They're probably just tweens being played in movie clips. So if a user clicked on button 2, you'd want only buttons 1,3, and 4 to reset...
Code:
stop();
button2.onRelease = function() {
if(button1._currentframe != 1) button1.gotoAndPlay("fadeOut");
if(button3._currentframe != 1) button3.gotoAndPlay("fadeOut");
if(button4._currentframe != 1) button4.gotoAndPlay("fadeOut");
}
...and so on for the other 3 button functions. You check the other buttons' _currentframe to make sure it isn't playing the "fade out" animation if it's already faded out.