The pipe the Sprixie Princess created doesn't lead to World 4. It leads to a small patch of land. A circus tent with Bowser's seal drops down and the pipe to World 4 is blocked off. There are circus lights all over. Go to right of the gondola for an item block with a Super Bell. The gondola takes you down to the arena, then disappears.
Walk towards the center and Hisstocrat appears. He's a snake wearing a cape and crown. Rocks will rain down from the sky. Dodge them, then more snakes will come out of the ground. They have plates on their heads to stand on, some of which have Super Bells. Climb up the snakes to reach Hisstocrat and land on his head for a hit. He'll go back in the ground and it will start raining rocks again. After two hits, some snakes will not have plates on their heads and will hurt you if you try to stand on them. Others will have cracked plates which will break when you jump off them. The snakes will hurt you after the plate shatters. Defeat Hisstocrat with the third hit and the Green Star will appear. The pipe will take you to the flagpole.
After you beat the level, the tent will tilt to one side and the pipe will no longer be blocked. Head to World 4.