Maths to calculate some x positions

Hey. I can’t get my head around the maths to work this out, I’ve been at it for hours.

I’m using stage listener to align some stuff based on the dimensions of the browser window stage rather than the actual stage set out in the fla. I have 4 buttons all of equal width that I want spaced out across the width of the browser. I’m quite sure the 1st button on the left can just have it’s ._x set to 0. Other than that I can’t get the rest to space out properly. The rest obviously need to overlap the first with the 4th button on the highest level and the middle 2 on the 2nd and 3rd highest. But whatver I try like stage.width/3 or 4 in the calculations just isn’t getting me anywhere and i dont even know if I need my reg point in the buttons left or right aligned or centred. Or a mixture?! I’m an idiot.

Thanks in advance for any pointers.