To access worlds 4 and 7 you first need to get a mini mushroom. You can get these at a toad house or in a game level. Then defeat the world 2 boss as mini mario. After the boss is defated you will notice a small opening at the bottom mini mario will fall in there and you will go to world 4. For world 7 defeat the world 5 boss as mini mario and mario will the go into the small opening at the bottom. You will then be at world 7.