Control Stacking order

I’m VERY new to flash. I have 4 buttons that stack on top of eachother. When they are moused over, I want the graphic to change, and for it to show on top of the other 3, but just while the mouse is over it. I’ve gone crazy trying to do this, and I’m completely out of ideas. I’ve seen the swapDepth’s function tossed around, but I don’t know how to use it. I’ve also seen some forums say that this can only be accomplished if it was a movieclip, but even when I change it to movieclip I can’t make it work. I’m using Flash CS4 and ActionScript 3.0. Can someone please hold my hand through this and help me? Please also specify where I put the Actionscript for this to work (on the main timeline or on the actions layer of the movieclip/button). Thanks so much!