The next door is on Stage 5 Level 6. To get the button you need the hammer or you'll never get the button. After you pass all of those wheel guys and the small area after that, there is a door on a little island right before the final door (practically right next to it). Just go through that door and use the hammer to bash through the metal block and the other blocks. All you have to do is go through that door and you've got your button.
The next button is sort of simple. You need the hammer for this one. (almost all of the rest of the buttons you'll need the hammer) In Stage 6 Part 1, go to the part right after the water part, there is a door before the door that goes to the next part of the level. Go through that door and use the hammer on the metal brick ( I think the block power can destroy the metal blocks also) and you'll have your button.
The next button you can use the hammer or the block power which I will tell you where that is in this level. When you start Stage 6 Level 2, go to the wall part that you have to destroy that bomb block to continue on. If you don't want to use the hammer, hit that block, fly up that hole and go through the black hole that is a door. When you go through, a block power guy (plus a extra life) will be there. Copy that guy for the block power. Continue on to the next part of that stage where there are cloud platforms.
When you get to the part where you see a door in a small room, on the left bottom of that room keep flying under that corner until your able to go through that door. Go through the door and there will be a wooden thing (plus two electric guys). Use the block power on the wooden thing and go through the door at the bottom. In that room there will be a button.