With a 4 mil wing, you will probably need 2 spars instead of one. If you only put in 1 spar, the top 4 mil will tend to crease at the spar, making an almost triangular airfoil. If you end up with this, it will probably have very poor low speed characteristics. Add a second spar about 3/4" to 1" back from the first and it will help that tremendously.

As far as glueing and folding. Do all of your folding first. Score your fold lines and fold the entire wing over into place. Like AtomHeart said, make a few practice runs first before you apply the glue. I would definately use medium CA and you would be surprised at how much time you really have. The main thing is once the two halves touch together, try not to let them move. You want to give it a couple minutes or so for the glue to set up before anything moves.

Hope this helps
Frank