When you get to world 4-2, go past the first set of '?' blocks and the elevator. Jump the hole and you will see a slightly lowered ceiling overhead. You can jump around beneath that area to reveal invisible blocks that form a staircase leading up toward the overhead bricks. Break those bricks and one of them will cause a beanstalk to appear. Climb the beanstalk to reach a secret bonus area. At the end of the bonus area, you will find pipes that allow you to warp directly to world 6, 7, or 8.