World 4 (New Super Mario Bros. 2)
From the Super Mario Wiki
World 4 is the fourth normal world appearing in New Super Mario Bros. 2. It is a snow-based world, and the levels are made up of snow or ice. The map music is the same one which was used in World 3 of New Super Mario Bros. Wii. This world is unlocked when a player completes World 3-Castle. This is also an optional world as the player can choose to go to World Flower instead.
There are 12 levels and five Toad Houses in this world, making it the longest world in New Super Mario Bros. 2 (World 3 has the same amount of levels, but two less Toad Houses). Of the 12 levels, eight levels are on the main path, which includes five normal levels, a Ghost House, and the Fortress and Castle. Of the remaining four levels, one is the Rainbow level, one is a level requiring payment of Star Coins and the other two are secret levels. Of the five Toad Houses, two are red, two are green and one is yellow. The boss of this world is Morton Koopa Jr., who is fought at the castle level.