After you pass the first floating panels in world 4-2 get to the edge of the left side and jump. You will hit a hidden block. Now skip a block over to the right and jump again. You will hit another. Do this again on the next space. Jump atop the hidden blocks and break the bricks all the way to your left. A vine will apear. climb it and continue. You wiil see the warp zone.

Warp 2, 3, 4

Continue through World 1-2 until you reach the second set of floating platforms. Jump on one and let it bring you high enough so you can jump onto the ledge to your right. After you are on the ledge you can continue to your right and your will reach the warp zone.

Order Mario Brothers game