Super Paper Mario

Go to the bottom of the Dotwood Tree and enter the Warp Pipe, if I'm wrong, please provide more information.

You must travel your way to the top of the tree, there will be a switch or something,press it, get back to the first yellow pipe inside the tree and it will take you to the door.

If you have already gone to the top of the tree and hit the switch, you need to go back to the bottom of this area. Go up the moving platform, and go left. The pipe there should take you up to the top.

The first pipe inside the tree takes you to the top of the tree.

1) Enter the first pipe to hit the first switch, and save if you like.
2) Go back via the pipe and work your way as high as you can until you find the switch that reveals the hidden door.
3) Return to the first pipe to get back up to the very top and the newly opened door, saving if you like.

You're close to the end of 4-3 once you get that far.