To warp to World 6,7 or 8 during World 4-2, go to the place where three blocks are can be seen (where a pipe can also be seen). Jump below the third block where you can reveal a secret block but be careful on jumping below the 1st block because you can't break it thus going to the secret warp world unless you are a small Mario. Then step on to it and break the 2nd and the 3rd block. After you have done this, go down, then jump again below the second block now (it should have been broken now). The position of the two revealed blocks must be a stair from right to left. Anyway, step on to the 2nd block then braek the 1st block and there you can see the vine going up. Climb until you reach the day light again. Get some coins if you wish and you can now see three pipes like in the secret warp Worlds 2,3 and 4. Go down your desired pipe.