Using the standard width of a 2x4 as an excuse to save 1/2 inch of concrete on your material bill is about the lamest excuse I have ever heard. For a slab on grade, you can always set your the top of your 2x4 on grade and kick a little dirt behind the bottom to keep the concrete from running underneath. If you're pouring on top of concrete, then you should be using a 2x6 with your grade snapped on it. I wonder if you guys are figuring 4" of concrete in your bid? If so, you are dishonest.