Spikey Problem
Kill Spike.

You can unlock World 4 by collecting all the stars that are scattered through the levels in World 1 to 3. This is located in World 4-5.

After you stomp on the first rocket, DON'T MOVE until after the walls disappear, because the lasers do a box pattern inside of the area that you're trapped in for a split second. After that focus on killing the rockets and surviving, as the time limit doesn't matter. You only have to pass by the rockets to launch them at him, so you don't have to stomp on them to fire them.

Be careful when you finally kill him, because I've been killing by his death animation by standing under him as he dies, so as soon as he gets his eyes crossed out, MOVE AWAY FROM UNDER HIM.