Swapping depths with a fade

Hello,
I want to swap depths on movieclips overlaying each other but instead of the new clip on rollover just popping up over the other one, i want it to fade into the top.

here is an attached .fla file in mx2004 format.

for those not using 2004, here is the code:

but1.onRollOver = but2.onRollOver = but3.onRollOver = but4.onRollOver = function() {
 	this.swapDepths(getNextHighestDepth());
 };

Also, is there a way to streamline this code so i dont have to type the button instance names every time i add another?