diff --git a/B4/Solutions/object_oriented_design/Media_streaming_service/features.text b/B4/Solutions/object_oriented_design/Media_streaming_service/features.text
new file mode 100644
index 00000000000..3f6e6ed0417
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/Media_streaming_service/features.text
@@ -0,0 +1,24 @@
+
+1. User Class:
+
+    a. Represents a user of the streaming service.
+    b. Can create playlists and receive content recommendations based on their playlists.
+        
+2. Subscription Class:
+
+    a. Represents a user's subscription type and monthly fee.
+        
+3. Media Class:
+
+    a. Represents a media item (e.g., a movie or song) with attributes like title, genre, and play count.
+    b. Includes a method to play the media, which increments the play count.
+
+4. Playlist Class:
+
+    a. Represents a user-created playlist that can hold multiple media items.
+    b. Includes methods to add media to the playlist and display the playlist.
+        
+5. MediaLibrary Class:
+
+    a. Manages the collection of media items available for streaming.
+    b. Includes methods to add media to the library and provide recommendations based on user playlists.
\ No newline at end of file
diff --git a/B4/Solutions/object_oriented_design/Media_streaming_service/media.py b/B4/Solutions/object_oriented_design/Media_streaming_service/media.py
new file mode 100644
index 00000000000..cdac909307e
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/Media_streaming_service/media.py
@@ -0,0 +1,92 @@
+class User:
+    def __init__(self, user_id, name, subscription):
+        self.user_id = user_id
+        self.name = name
+        self.subscription = subscription
+        self.playlists = []
+
+    def create_playlist(self, playlist_name):
+        playlist = Playlist(playlist_name)
+        self.playlists.append(playlist)
+        print(f"Playlist '{playlist_name}' created for {self.name}.")
+
+    def recommend_content(self, media_library):
+        recommendations = media_library.get_recommendations(self)
+        print(f"Recommendations for {self.name}: {', '.join(recommendations)}")
+
+class Subscription:
+    def __init__(self, subscription_type, monthly_fee):
+        self.subscription_type = subscription_type
+        self.monthly_fee = monthly_fee
+
+class Media:
+    def __init__(self, media_id, title, genre):
+        self.media_id = media_id
+        self.title = title
+        self.genre = genre
+        self.play_count = 0
+
+    def play(self):
+        self.play_count += 1
+        print(f"Playing '{self.title}'.")
+
+class Playlist:
+    def __init__(self, name):
+        self.name = name
+        self.media_items = []
+
+    def add_media(self, media):
+        self.media_items.append(media)
+        print(f"Added '{media.title}' to playlist '{self.name}'.")
+
+    def show_playlist(self):
+        print(f"Playlist '{self.name}': {[media.title for media in self.media_items]}")
+
+class MediaLibrary:
+    def __init__(self):
+        self.media_collection = []
+
+    def add_media(self, media):
+        self.media_collection.append(media)
+        print(f"Media '{media.title}' added to the library.")
+
+    def get_recommendations(self, user):
+        # Simple recommendation based on genres of user's playlists
+        recommended = []
+        for playlist in user.playlists:
+            for media in playlist.media_items:
+                if media.genre not in recommended:
+                    recommended.append(media.genre)
+        
+        # Return a list of media titles from the library that match recommended genres
+        recommendations = [media.title for media in self.media_collection if media.genre in recommended]
+        return recommendations[:5]  # Limit to 5 recommendations
+
+# Example Usage
+# Create media library and add media
+media_library = MediaLibrary()
+media_library.add_media(Media(1, "Inception", "Sci-Fi"))
+media_library.add_media(Media(2, "Titanic", "Romance"))
+media_library.add_media(Media(3, "The Matrix", "Sci-Fi"))
+media_library.add_media(Media(4, "The Godfather", "Crime"))
+media_library.add_media(Media(5, "Interstellar", "Sci-Fi"))
+
+# Create a subscription
+basic_subscription = Subscription("Basic", 9.99)
+
+# Create a user
+user1 = User(1, "Alice", basic_subscription)
+
+# User creates a playlist and adds media
+user1.create_playlist("Favorites")
+user1.playlists[0].add_media(media_library.media_collection[0])  # Inception
+user1.playlists[0].add_media(media_library.media_collection[2])  # The Matrix
+
+# Show the user's playlist
+user1.playlists[0].show_playlist()
+
+# Get recommendations for the user
+user1.recommend_content(media_library)
+
+# Play a media item
+media_library.media_collection[0].play()  # Play Inception
diff --git a/B4/Solutions/object_oriented_design/banking_system/banking.py b/B4/Solutions/object_oriented_design/banking_system/banking.py
new file mode 100644
index 00000000000..18a8e4e760e
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/banking_system/banking.py
@@ -0,0 +1,62 @@
+class Account:
+    def __init__(self, account_number, account_holder, balance=0):
+        self.account_number = account_number
+        self.account_holder = account_holder
+        self.balance = balance
+
+    def deposit(self, amount):
+        self.balance += amount
+        print(f"Deposited {amount}. New balance: {self.balance}")
+
+    def withdraw(self, amount):
+        if amount > self.balance:
+            print("Insufficient funds.")
+        else:
+            self.balance -= amount
+            print(f"Withdrawn {amount}. New balance: {self.balance}")
+
+    def get_balance(self):
+        return self.balance
+
+class Transaction:
+    def __init__(self, transaction_id, account, amount, transaction_type):
+        self.transaction_id = transaction_id
+        self.account = account
+        self.amount = amount
+        self.transaction_type = transaction_type
+
+    def execute(self):
+        if self.transaction_type == "deposit":
+            self.account.deposit(self.amount)
+        elif self.transaction_type == "withdraw":
+            self.account.withdraw(self.amount)
+        else:
+            print("Invalid transaction type.")
+
+class Bank:
+    def __init__(self):
+        self.accounts = {}
+
+    def create_account(self, account_number, account_holder):
+        if account_number not in self.accounts:
+            self.accounts[account_number] = Account(account_number, account_holder)
+            print(f"Account created for {account_holder}.")
+        else:
+            print("Account already exists.")
+
+    def get_account(self, account_number):
+        return self.accounts.get(account_number, None)
+
+# Example Usage
+bank = Bank()
+bank.create_account("123456", "Alice")
+bank.create_account("789012", "Bob")
+
+alice_account = bank.get_account("123456")
+transaction1 = Transaction(1, alice_account, 1000, "deposit")
+transaction1.execute()
+
+transaction2 = Transaction(2, alice_account, 500, "withdraw")
+transaction2.execute()
+
+print(f"Alice's balance: {alice_account.get_balance()}")
diff --git a/B4/Solutions/object_oriented_design/banking_system/features.text b/B4/Solutions/object_oriented_design/banking_system/features.text
new file mode 100644
index 00000000000..9cedb762c5a
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/banking_system/features.text
@@ -0,0 +1,9 @@
+Banking System:
+
+1. Account: Represents a bank account, with methods to deposit, 
+    withdraw, and check the balance.
+
+2. Transaction: Represents a transaction, which can either be a 
+    deposit or withdrawal.
+
+3. Bank: Manages accounts and allows for account creation.
\ No newline at end of file
diff --git a/B4/Solutions/object_oriented_design/food_delivery/features.text b/B4/Solutions/object_oriented_design/food_delivery/features.text
new file mode 100644
index 00000000000..8b7e827ffb1
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/food_delivery/features.text
@@ -0,0 +1,14 @@
+Features of this Food Delivery System:
+
+1. Customer Class: Represents a customer with attributes like name, address, and phone number.
+
+2. Restaurant Class: Represents a restaurant with a menu. It allows the system to show available 
+   items and get the price of food.
+
+3. DeliveryPerson Class: Represents a delivery person who is assigned to deliver an order.
+
+4. Order Class: Represents an order with details like the customer, restaurant, and the food 
+    item ordered. It also includes the price of the order.
+    
+5. FoodDeliverySystem Class: This is the core of the system. It manages customers, restaurants, 
+    delivery personnel, and the order placement process. It assigns a delivery person to each order.
\ No newline at end of file
diff --git a/B4/Solutions/object_oriented_design/food_delivery/food_delivery.py b/B4/Solutions/object_oriented_design/food_delivery/food_delivery.py
new file mode 100644
index 00000000000..4f65aa547ce
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/food_delivery/food_delivery.py
@@ -0,0 +1,134 @@
+class Customer:
+    def __init__(self, name, address, phone):
+        self.name = name
+        self.address = address
+        self.phone = phone
+
+    def __str__(self):
+        return f"Customer: {self.name}, Address: {self.address}, Phone: {self.phone}"
+
+
+class Restaurant:
+    def __init__(self, name, menu):
+        self.name = name
+        self.menu = menu  # menu is a dictionary with item names as keys and prices as values
+
+    def show_menu(self):
+        print(f"Menu for {self.name}:")
+        for item, price in self.menu.items():
+            print(f"{item}: ${price:.2f}")
+
+    def get_price(self, item):
+        return self.menu.get(item, None)
+
+    def __str__(self):
+        return f"Restaurant: {self.name}"
+
+
+class DeliveryPerson:
+    def __init__(self, name):
+        self.name = name
+
+    def deliver(self, customer, order):
+        print(f"{self.name} is delivering {order.food_item} to {customer.name} at {customer.address}")
+
+    def __str__(self):
+        return f"Delivery Person: {self.name}"
+
+
+class Order:
+    def __init__(self, customer, restaurant, food_item):
+        self.customer = customer
+        self.restaurant = restaurant
+        self.food_item = food_item
+        self.price = self.restaurant.get_price(food_item)
+
+    def display_order_details(self):
+        print(f"Order Details: \nCustomer: {self.customer.name}\nFood Item: {self.food_item}\n"
+              f"Price: ${self.price:.2f}\nRestaurant: {self.restaurant.name}")
+
+    def __str__(self):
+        return f"Order: {self.food_item} from {self.restaurant.name} for {self.customer.name}"
+
+
+class FoodDeliverySystem:
+    def __init__(self):
+        self.customers = []
+        self.restaurants = []
+        self.delivery_people = []
+
+    def add_customer(self, customer):
+        self.customers.append(customer)
+
+    def add_restaurant(self, restaurant):
+        self.restaurants.append(restaurant)
+
+    def add_delivery_person(self, delivery_person):
+        self.delivery_people.append(delivery_person)
+
+    def place_order(self, customer, restaurant, food_item):
+        if food_item not in restaurant.menu:
+            print(f"Sorry, {food_item} is not available at {restaurant.name}.")
+            return None
+
+        order = Order(customer, restaurant, food_item)
+        order.display_order_details()
+        delivery_person = self.assign_delivery_person()
+        if delivery_person:
+            delivery_person.deliver(customer, order)
+        return order
+
+    def assign_delivery_person(self):
+        if not self.delivery_people:
+            print("No delivery person available at the moment.")
+            return None
+        # Assign the first available delivery person
+        return self.delivery_people[0]
+
+    def show_restaurants(self):
+        for restaurant in self.restaurants:
+            print(restaurant)
+
+    def show_customers(self):
+        for customer in self.customers:
+            print(customer)
+
+
+# Example usage
+if __name__ == "__main__":
+    # Initialize the system
+    delivery_system = FoodDeliverySystem()
+
+    # Create some customers
+    customer1 = Customer("Alice", "123 Main St", "555-1234")
+    customer2 = Customer("Bob", "456 Oak St", "555-5678")
+
+    # Add customers to the system
+    delivery_system.add_customer(customer1)
+    delivery_system.add_customer(customer2)
+
+    # Create some restaurants with menus
+    pizza_place = Restaurant("Pizza Place", {"Pizza": 10.99, "Burger": 7.99, "Pasta": 8.99})
+    sushi_spot = Restaurant("Sushi Spot", {"Sushi Roll": 12.99, "Tempura": 9.99, "Miso Soup": 3.99})
+
+    # Add restaurants to the system
+    delivery_system.add_restaurant(pizza_place)
+    delivery_system.add_restaurant(sushi_spot)
+
+    # Create a delivery person
+    delivery_person1 = DeliveryPerson("John")
+
+    # Add delivery person to the system
+    delivery_system.add_delivery_person(delivery_person1)
+
+    # Display available restaurants
+    print("Available Restaurants:")
+    delivery_system.show_restaurants()
+
+    # Customer places an order
+    print("\nCustomer 1 places an order:")
+    delivery_system.place_order(customer1, pizza_place, "Pizza")
+
+    # Another customer places an order
+    print("\nCustomer 2 places an order:")
+    delivery_system.place_order(customer2, sushi_spot, "Sushi Roll")
diff --git a/B4/Solutions/object_oriented_design/hotel_booking/features.text b/B4/Solutions/object_oriented_design/hotel_booking/features.text
new file mode 100644
index 00000000000..3ac297b223f
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/hotel_booking/features.text
@@ -0,0 +1,22 @@
+Explanation
+
+1. Room Class:
+
+    a. Represents a hotel room with attributes for room number, type, price, and availability.
+    b. Methods include book_room (to book the room) and release_room (to make the room available again).
+        
+2. Customer Class:
+
+    a. Represents a customer who can book rooms.
+    b. Includes a method make_booking that attempts to book a room and create a Booking object if successful.
+        
+3. Booking Class:
+
+    a. Represents a booking made by a customer.
+    b. Calculates total price based on the duration of the stay and the room price.
+    c. Includes a method cancel_booking to release the room and remove the booking from the customer’s list.
+        
+4. Hotel Class:
+
+    a. Manages the collection of rooms.
+    b. Provides methods to add rooms, search for available rooms, and display all rooms.
\ No newline at end of file
diff --git a/B4/Solutions/object_oriented_design/hotel_booking/hotel.py b/B4/Solutions/object_oriented_design/hotel_booking/hotel.py
new file mode 100644
index 00000000000..b26c359a796
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/hotel_booking/hotel.py
@@ -0,0 +1,96 @@
+class Room:
+    def __init__(self, room_number, room_type, price):
+        self.room_number = room_number
+        self.room_type = room_type
+        self.price = price
+        self.is_available = True
+
+    def book_room(self):
+        if self.is_available:
+            self.is_available = False
+            return True
+        return False
+
+    def release_room(self):
+        self.is_available = True
+
+class Customer:
+    def __init__(self, customer_id, name):
+        self.customer_id = customer_id
+        self.name = name
+        self.bookings = []
+
+    def make_booking(self, room, check_in, check_out):
+        if room.book_room():
+            booking = Booking(self, room, check_in, check_out)
+            self.bookings.append(booking)
+            print(f"Booking successful for {self.name} in room {room.room_number}.")
+            return booking
+        print(f"Room {room.room_number} is not available.")
+        return None
+
+class Booking:
+    def __init__(self, customer, room, check_in, check_out):
+        self.customer = customer
+        self.room = room
+        self.check_in = check_in
+        self.check_out = check_out
+        self.total_price = self.calculate_price()
+
+    def calculate_price(self):
+        # Assuming check_in and check_out are datetime.date objects
+        stay_duration = (self.check_out - self.check_in).days
+        return stay_duration * self.room.price
+
+    def cancel_booking(self):
+        self.room.release_room()
+        self.customer.bookings.remove(self)
+        print(f"Booking for room {self.room.room_number} has been canceled.")
+
+class Hotel:
+    def __init__(self):
+        self.rooms = []
+
+    def add_room(self, room_number, room_type, price):
+        room = Room(room_number, room_type, price)
+        self.rooms.append(room)
+        print(f"Room {room_number} added.")
+
+    def search_rooms(self):
+        available_rooms = [room for room in self.rooms if room.is_available]
+        return available_rooms
+
+    def display_rooms(self):
+        for room in self.rooms:
+            status = "Available" if room.is_available else "Booked"
+            print(f"Room {room.room_number}: {room.room_type}, Price: ${room.price}, Status: {status}")
+
+# Example Usage
+from datetime import date
+
+hotel = Hotel()
+hotel.add_room(101, "Single", 100)
+hotel.add_room(102, "Double", 150)
+hotel.add_room(103, "Suite", 250)
+
+# Display all rooms
+hotel.display_rooms()
+
+# Create a customer
+customer1 = Customer(1, "Alice")
+
+# Search for available rooms
+available_rooms = hotel.search_rooms()
+if available_rooms:
+    # Customer books the first available room
+    booking1 = customer1.make_booking(available_rooms[0], date(2024, 10, 25), date(2024, 10, 28))
+
+# Display bookings for the customer
+print(f"Bookings for {customer1.name}: {[booking.room.room_number for booking in customer1.bookings]}")
+
+# Cancel booking
+if booking1:
+    booking1.cancel_booking()
+
+# Display rooms after cancellation
+hotel.display_rooms()
diff --git a/B4/Solutions/object_oriented_design/ride_sharing/features.text b/B4/Solutions/object_oriented_design/ride_sharing/features.text
new file mode 100644
index 00000000000..03f7f4c594b
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/ride_sharing/features.text
@@ -0,0 +1,10 @@
+Ride-Sharing Service:
+
+1. User: Represents a user who can request rides.
+
+2. Driver: Represents a driver who can accept rides.
+
+3. Ride: Represents a ride request, including the user and locations.
+
+4. RideSharingService: Manages users and drivers and includes logic 
+    to match rides.
\ No newline at end of file
diff --git a/B4/Solutions/object_oriented_design/ride_sharing/ride.py b/B4/Solutions/object_oriented_design/ride_sharing/ride.py
new file mode 100644
index 00000000000..ffca8ba2a1e
--- /dev/null
+++ b/B4/Solutions/object_oriented_design/ride_sharing/ride.py
@@ -0,0 +1,64 @@
+import random
+
+class User:
+    def __init__(self, user_id, name):
+        self.user_id = user_id
+        self.name = name
+        self.rides = []
+
+    def request_ride(self, pickup_location, dropoff_location):
+        ride = Ride(self, pickup_location, dropoff_location)
+        return ride
+
+class Driver:
+    def __init__(self, driver_id, name):
+        self.driver_id = driver_id
+        self.name = name
+        self.rides = []
+
+    def accept_ride(self, ride):
+        self.rides.append(ride)
+        ride.driver = self
+        print(f"Driver {self.name} accepted ride from {ride.user.name}.")
+
+class Ride:
+    def __init__(self, user, pickup_location, dropoff_location):
+        self.user = user
+        self.pickup_location = pickup_location
+        self.dropoff_location = dropoff_location
+        self.driver = None
+        self.ride_id = random.randint(1000, 9999)  # Random ID for ride
+
+class RideSharingService:
+    def __init__(self):
+        self.drivers = []
+        self.users = []
+
+    def register_driver(self, driver_id, name):
+        driver = Driver(driver_id, name)
+        self.drivers.append(driver)
+        print(f"Driver {name} registered.")
+
+    def register_user(self, user_id, name):
+        user = User(user_id, name)
+        self.users.append(user)
+        print(f"User {name} registered.")
+
+    def match_ride(self, ride):
+        if self.drivers:
+            selected_driver = random.choice(self.drivers)  # Simple random matching
+            selected_driver.accept_ride(ride)
+        else:
+            print("No drivers available.")
+
+# Example Usage
+ride_sharing_service = RideSharingService()
+ride_sharing_service.register_driver(1, "John")
+ride_sharing_service.register_driver(2, "Jane")
+
+ride_sharing_service.register_user(101, "Alice")
+ride_sharing_service.register_user(102, "Bob")
+
+alice = ride_sharing_service.users[0]
+ride_request = alice.request_ride("Location A", "Location B")
+ride_sharing_service.match_ride(ride_request)
diff --git a/B4/Solutions/system_design/Video_conferencing/features.text b/B4/Solutions/system_design/Video_conferencing/features.text
new file mode 100644
index 00000000000..be584c4c2c5
--- /dev/null
+++ b/B4/Solutions/system_design/Video_conferencing/features.text
@@ -0,0 +1,5 @@
+Video Conferencing System:
+
+1. User: Represents a user in the system.
+
+2. Meeting: Handles participant management and audio/video streaming.
\ No newline at end of file
diff --git a/B4/Solutions/system_design/Video_conferencing/video.py b/B4/Solutions/system_design/Video_conferencing/video.py
new file mode 100644
index 00000000000..34f001ea335
--- /dev/null
+++ b/B4/Solutions/system_design/Video_conferencing/video.py
@@ -0,0 +1,60 @@
+import random
+import threading
+import time
+
+class User:
+    def __init__(self, user_id, name):
+        self.user_id = user_id
+        self.name = name
+
+class Meeting:
+    def __init__(self, meeting_id):
+        self.meeting_id = meeting_id
+        self.participants = []
+        self.is_active = True
+
+    def add_participant(self, user):
+        self.participants.append(user)
+        print(f"{user.name} joined the meeting {self.meeting_id}.")
+
+    def remove_participant(self, user):
+        self.participants.remove(user)
+        print(f"{user.name} left the meeting {self.meeting_id}.")
+
+    def stream_audio_video(self):
+        # Simulated audio/video streaming
+        while self.is_active:
+            print(f"Streaming audio/video in meeting {self.meeting_id}...")
+            time.sleep(2)
+
+    def end_meeting(self):
+        self.is_active = False
+        print(f"Meeting {self.meeting_id} has ended.")
+
+class VideoConferencingSystem:
+    def __init__(self):
+        self.meetings = {}
+
+    def create_meeting(self):
+        meeting_id = random.randint(1000, 9999)
+        meeting = Meeting(meeting_id)
+        self.meetings[meeting_id] = meeting
+        print(f"Meeting {meeting_id} created.")
+        return meeting
+
+# Example Usage
+vcs = VideoConferencingSystem()
+meeting = vcs.create_meeting()
+
+user1 = User(1, "Alice")
+user2 = User(2, "Bob")
+
+meeting.add_participant(user1)
+meeting.add_participant(user2)
+
+stream_thread = threading.Thread(target=meeting.stream_audio_video)
+stream_thread.start()
+
+time.sleep(5)
+meeting.end_meeting()
+stream_thread.join()
diff --git a/B4/Solutions/system_design/payment_processing/features.text b/B4/Solutions/system_design/payment_processing/features.text
new file mode 100644
index 00000000000..45a71151251
--- /dev/null
+++ b/B4/Solutions/system_design/payment_processing/features.text
@@ -0,0 +1,7 @@
+Payment Processing System:
+
+1. User: Represents a user with a balance.
+
+2. Transaction: Represents a payment transaction between users.
+
+3. PaymentProcessingSystem: Processes transactions and handles fund transfers.
\ No newline at end of file
diff --git a/B4/Solutions/system_design/payment_processing/payment.py b/B4/Solutions/system_design/payment_processing/payment.py
new file mode 100644
index 00000000000..cf1f88d8983
--- /dev/null
+++ b/B4/Solutions/system_design/payment_processing/payment.py
@@ -0,0 +1,34 @@
+import random
+
+class User:
+    def __init__(self, user_id, name, balance):
+        self.user_id = user_id
+        self.name = name
+        self.balance = balance
+
+class Transaction:
+    def __init__(self, sender, receiver, amount):
+        self.transaction_id = random.randint(1000, 9999)
+        self.sender = sender
+        self.receiver = receiver
+        self.amount = amount
+
+class PaymentProcessingSystem:
+    def process_payment(self, transaction):
+        if transaction.sender.balance >= transaction.amount:
+            transaction.sender.balance -= transaction.amount
+            transaction.receiver.balance += transaction.amount
+            print(f"Transaction {transaction.transaction_id} successful: {transaction.sender.name} paid {transaction.receiver.name} ${transaction.amount}.")
+        else:
+            print(f"Transaction {transaction.transaction_id} failed: Insufficient funds.")
+
+# Example Usage
+user1 = User(1, "Alice", 100)
+user2 = User(2, "Bob", 50)
+
+payment_system = PaymentProcessingSystem()
+transaction = Transaction(user1, user2, 30)
+payment_system.process_payment(transaction)
+
+print(f"{user1.name}'s balance: ${user1.balance}")
+print(f"{user2.name}'s balance: ${user2.balance}")
diff --git a/B4/Solutions/system_design/realtime_collaborative_editing/collab.py b/B4/Solutions/system_design/realtime_collaborative_editing/collab.py
new file mode 100644
index 00000000000..737699f44e3
--- /dev/null
+++ b/B4/Solutions/system_design/realtime_collaborative_editing/collab.py
@@ -0,0 +1,36 @@
+import threading
+
+class Document:
+    def __init__(self, title):
+        self.title = title
+        self.content = ""
+        self.lock = threading.Lock()
+
+    def edit(self, new_content):
+        with self.lock:
+            self.content = new_content
+            print(f"Document '{self.title}' updated: {self.content}")
+
+class User:
+    def __init__(self, user_id, name):
+        self.user_id = user_id
+        self.name = name
+
+    def edit_document(self, document, new_content):
+        print(f"{self.name} is editing the document '{document.title}'...")
+        document.edit(new_content)
+
+# Example Usage
+document = Document("Project Plan")
+user1 = User(1, "Alice")
+user2 = User(2, "Bob")
+
+# Simulating concurrent edits
+thread1 = threading.Thread(target=user1.edit_document, args=(document, "Draft by Alice."))
+thread2 = threading.Thread(target=user2.edit_document, args=(document, "Draft by Bob."))
+
+thread1.start()
+thread2.start()
+
+thread1.join()
+thread2.join()
diff --git a/B4/Solutions/system_design/realtime_collaborative_editing/features.rext b/B4/Solutions/system_design/realtime_collaborative_editing/features.rext
new file mode 100644
index 00000000000..a66967996d7
--- /dev/null
+++ b/B4/Solutions/system_design/realtime_collaborative_editing/features.rext
@@ -0,0 +1,7 @@
+1. Document: Represents a document being edited, with content locked for 
+    safe concurrent editing.
+
+2. User: Represents a user editing the document.
+
+3. Each user can edit the document concurrently, with proper locking to prevent 
+    data corruption.
\ No newline at end of file
diff --git a/B4/Solutions/system_design/smart_home/features.p[y b/B4/Solutions/system_design/smart_home/features.p[y
new file mode 100644
index 00000000000..614c831beac
--- /dev/null
+++ b/B4/Solutions/system_design/smart_home/features.p[y
@@ -0,0 +1,5 @@
+Smart Home System:
+
+1. SmartDevice: Represents a smart device with on/off control.
+
+2. SmartHomeSystem: Manages devices and controls their states.
\ No newline at end of file
diff --git a/B4/Solutions/system_design/smart_home/smart_home.py b/B4/Solutions/system_design/smart_home/smart_home.py
new file mode 100644
index 00000000000..0442449d894
--- /dev/null
+++ b/B4/Solutions/system_design/smart_home/smart_home.py
@@ -0,0 +1,44 @@
+class SmartDevice:
+    def __init__(self, device_id, device_type):
+        self.device_id = device_id
+        self.device_type = device_type
+        self.is_on = False
+
+    def turn_on(self):
+        self.is_on = True
+        print(f"{self.device_type} {self.device_id} turned on.")
+
+    def turn_off(self):
+        self.is_on = False
+        print(f"{self.device_type} {self.device_id} turned off.")
+
+class SmartHomeSystem:
+    def __init__(self):
+        self.devices = {}
+
+    def add_device(self, device):
+        self.devices[device.device_id] = device
+        print(f"Added {device.device_type} {device.device_id}.")
+
+    def control_device(self, device_id, action):
+        device = self.devices.get(device_id)
+        if device:
+            if action == "on":
+                device.turn_on()
+            elif action == "off":
+                device.turn_off()
+            else:
+                print("Invalid action.")
+        else:
+            print("Device not found.")
+
+# Example Usage
+smart_home = SmartHomeSystem()
+light = SmartDevice(1, "Light")
+thermostat = SmartDevice(2, "Thermostat")
+
+smart_home.add_device(light)
+smart_home.add_device(thermostat)
+
+smart_home.control_device(1, "on")  # Turn on the light
+smart_home.control_device(2, "off")  # Turn off the thermostat
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