This is how you unlock world 4 and 7. You play the castle of level 2, but you have to be mini, like not just small, mini Mario/Luigi. Then you have to beat the level as a mini dude, then it unlocks the world, for world 7 you play world 6's castle, to unlock one half of it, then play world 8's castle which unlocks the other half of world 7. That's how