World 4 (Super Mario 3D Land)
From the Super Mario Wiki
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World 4 is the fourth world contained in Super Mario 3D Land. This world takes place in a lime-green grassland with mushrooms of varying sizes bouncing up and down in the background. World 4 contains six levels and a Mystery Box. It is also the first encounter with Pom Pom, the boss of this World.
The two possible contents of this world's Mystery Box are: