Locate the jumping platforms at the end of level 1-3. Jump to the top of the screen. Near the top are three jumping platforms in a triangular arrangement. Jump off the lower right platform and punch the air. The sky will begin to open. Hit this same location until a black hole appears. Jump into the hole to enter the warp area. Slide down the steel ramp until two pipes appear. The first pipe leads to level 5-1 and the second pipe leads to level 4-1.