The New Super Mario Brothers. How do you get to World 4?

Answer 1

in world 2, you will notice that after you beat the boss at the castle, you pass the door to see Peach get wisked away by Baby Bowser. if you pay close attention, you will notice that the floor has a path under it, fit for someone who is really tiny. if you can get there while that tiny, you will be able to take that path and go to world 4. in other words, beat the world 2 boss while Mini to get to world 4. (same goes with world 7, just beat world 5 while Mini)