World 4 (Super Mario 3D Land)
From the Super Mario Wiki
World 4 is the fourth world contained in Super Mario 3D Land. This world is a pale green meadow with small, medium and gigantic mushrooms. World 4 contains 6 levels and a Mystery Box. It is also the first encounter with Pom Pom, the boss of this World.
 Mystery Box
The possible contents of this world's Mystery Box are:
 New Enemies