In World 4-2, as you head right in the underground segment, you will see seven ? Blocks (2 over 2, with 3 to the side). Past the next gap are some blocks on the roof. You can bounce around near here to reveal some blocks, which you can use to hit the already-revealed bricks to reveal a vine. Climb this vine to go up and into a linear platforming area leading to a warp zone, which takes you to World 6.