An all 4 mil wing will be fine. It'll be just slightly heavier is all.
I use medium CA when gluing the top half of the wing to the spar. Thin CA soaks into the yardstick too fast to get the top folded down. You still have to work kind of quick though. The best thing to do is do a practice fold and hold, with no glue so you can get an idea of what you need to do. If all you have is thin CA you can tack it down and then put the wing on it's end and run a bead of CA down the spar. Just make sure you have something besides your foot or new carpet on the other end of the wing.