....and 4" minimum MEANS 4" minimum. NOT 3.5" used with 2 x 4s for framing up. You have to watch it, because a lot of contractors will call a 3.5" pout a 4" pour when that extra 1/2" matters.
EDIT: FWIW, I've never personally witnessed any business pouring concrete using 2x4s unless they were stacked up...and if I did, they wouldn't pour for me
