World 4 is the fourth world of the game New Super Mario Bros. Wii, and is controlled by Wendy O. Koopa. It features several tropical islands, therefore, most of its levels are water-themed and make rather good use of the Penguin Suit, which can glide over water and allow Mario to swim better. Also, on the map is a bridge guarded by a Porcu-Puffer that can jump up and make Mario and the gang endure an Enemy Course. This is also the first world to have a level on an Airship, like the ones in Super Mario Bros. 3.
World 4 also appears in Mario & Sonic at the London 2012 Olympic Games under the name "Cheep Cheep River" as the location of the Dream Rafting event. Porcu-Puffers, Thwomps, Cheep-Cheeps, and Bob-ombs appear in it, as well as the addition of Gooper Blooper. Also, Wendy (the boss of this level) appears as a costume for the player's Mii.
 Level Previews
The beginning area of World 4.
 New Enemies