New Super Mario Bros./World 4

World 4 is one of two possible worlds that come after World 2, as well as one of the two secret worlds that require a special requirement to unlock. Compared to its counterpart, World 3, it has about the same amount of levels but less split paths, making the World seem overall longer. It also has much more Toad Houses, but most of them are locked behind Star Coin signs.

World 4-1
During the Dorrie ride, wait until you approach the third set of brick blocks, which also happens to be the longest. Get on the brick blocks and stand in the middle. Jump directly up to cause a vine to pop out, which leads to a platform with a pipe in the canopy of the jungle. When you emerge, jump on to the hills and try to leap over the trio of Scuttlebugs to reach the top of the red flagpole.

This secret exit lets you bypass two levels to get to World 4-Tower, granting access to a green Toad House along the way.

World 4-Ghost
You'll need a Mini Mushroom to get to this secret exit. After hitting the second red switch, head on to the pipe with red blocks beside its exit. Instead of heading through and progressing as normal, hop on the top of the warp pipe. Wall jump off the pipe and to the right, where there is a ledge that is otherwise inaccessible without the floaty jumps of Mini Mario/Luigi. Jump up and into the gap above. You will emerge in another room. Ignore the door first and keep using Mini Mario's wall jumps to get the last Star Coin. Fall back down and go through the door to be taken to the red flag.

This hidden exit leads to the World 4 Cannon, which blast you far above the canopy and into the cloudy World 7.