For level 4:
Go on to the final castle on level 2 and defeat bowser as mini mario. It does work i have done it.
For level 7:
Go on to the final castle on level 5 and defeat bowser as mini mario they both really work as i have tried it and now have all levels.
How to get to 8-1 easily:
At 4-2 get to the first platform and the first block you see hit it and a vine will pop out and to get to climb the vine under it will be a invisible coin block hit it and then jump on it and climb up the vine (note) this does not work with mega mario or fire mario.