World 4 is the fourth world that Mario and Luigi venture through in New Super Mario Bros. with a jungle theme. This world features many jungle-themed levels along with toxic water levels. Strangely, when Mario or Luigi touches the toxic water, they lose a life regardless of which form they are.
This world is accessible via defeating Mummipokey in World 2 and isn't required to complete the game. Instead, Mario and Luigi can go through World 3 in order to advance throughout the game. This world features newly-introduced enemies such as Wigglers and a helper called Dorrie who helps Mario and Luigi cross the poisonous purple water. Like all of the other worlds, Bowser Jr. is the mini-boss here and Mega Goomba is the boss in this world.