Can You Get a BMX Bike in GTA IV (Xbox 360)? No

No, BMX bicycles are not available in Grand Theft Auto IV on any platform, including Xbox 360. The game's vehicle roster focuses on motorized transport.

Two‑Wheeled Alternatives in Liberty City

• Motorcycles: Sports bikes (NRG 900), cruisers (Zombie, Hexer), and dirt bikes (Sanchez) offer high speed and agility.
• Scooters: The Faggio is a slower, maneuverable option found in dense urban areas.

If you specifically want a BMX gameplay experience, consider titles built around it, such as the Tony Hawk's Pro Skater series (which includes BMX modes), Dave Mirra Freestyle BMX, or BMX XXX.