Not sure what you are after
so that one button changes the input
but this may give you an idea. http://www.picbasic.co.uk/forum/showthread.php?t=8602

Or maybe use "count" routine. every time the button is pressed the counter increases. If the count VAR is 1 do something. If count VAR is 2 do something else. If count VAR > 4 then count VAR = 0.
Is that what you need?