B-4

B-4 can refer to the following:
 * World B-4 from Super Mario Bros.: The Lost Levels.
 * Room B-4 from Mario vs. Donkey Kong: Minis March Again!.
 * Pool Party, mission B-4 from Luigi's Mansion: Dark Moon.
 * Level B-4 from Mario vs. Donkey Kong: Tipping Stars.

B-4