How do you get into world 4 and 7 on new Super Mario Brothers?

Answer 1

Visit the castle in world 2 or 5, find a mini mushroom in a ? block or use it if you have it as an item to fight the bosses. You must defeat the bosses in world 3-castle and 6-castle as mini mario (or luigi). After collecting the key and adding your bonus, the gate on the right side will open up. If you'd done the castle before as regular mario or luigi, you'll see your character question why peach isn't there. If this is your first time completing the castle and you defeat the boss as mini mario or luigi, your character will see bowser jr. making noises at the character and running off while your character falls into a little hole and turns the sign when he gets near from "W3" to "W4" or "W6" to "W7." If you'd done the castle before as regular mario or luigi and you want to unlock world 4 or 7, defeat the boss as mini mario or luigi. This time, your character will question why peach and bowser jr. aren't there. And he will fall into the little hole and change the sign as stated above. Then you will unlock world 4 or 7 and get an opportunity to get big in the first world of 4 or 7.