The Cat Suit is required for this cheat, so have one handy in the inventory or nab it in the area. After exiting the underground portion (with the checkpoint flag) you will come out of a warp pipe with a clear mystery box. Run up the wall that it is touching to reach the top of the waterfall. After jumping across the poisonous water, there will be red warp pipe. This will warp you to World 5.
