World 4 (New Super Mario Bros.)
From the Super Mario Wiki
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World 4 is an alternate world found in the game New Super Mario Bros.. This world is mainly jungle-based, and features poison, which defeats the player with a single touch; regardless of what form they take.
This world is accessible by defeating Mummipokey from World 2 in Mini Mario form. World 4 introduces a lot of new enemies, such as Wigglers and Scuttlebugs. It also introduces Dorrie: a friendly creature who helps the Mario bros. travel along the toxic water. As with all the other worlds, Bowser Jr. is fought here in World 4-. The boss of this world is Mega Goomba, and defeating him will lead the player to World 5. If the player were to use World 4-Cannon, unlocked via World 4-'s secret exit, then they'd reach World 7.