flash actionscript: I think what I want is reasonably basic, but I can't work it right!

I have my screen and there are four seperate movies on it. When these movies
are dragged onto the correct movieclip, they run themselves so that they appear
to drop off stage.

I want a button to become fully opaque when all four of these movies have
played through to their last frame (frame 20), but I'm totally lost...could
anyone help?!

PLEASE!

the easiest solution would be to add the following code to the last frame of
each of your four movieclips:



if(!_root.movieComplete){
_root.movieComplete=1;
} else {
_root.movieComplete++;
if(_root.movieComplete==4){
_root.yourButton._alpha=100; // make sure to use your button's instance name.
}
}