Hey!
It turned out to be the valve body. The tranny guy started replacing one thing, then putting it back together, testing it, then taking it apart and swapping the next thing out. Finally the New Valve body turned out to be bad.
Hey buddy. I was just viewing an old transmission thread and I seem to be having the same problem you are (4th gear starts). Mine is intermentent and also started happening after a recent trans rebuild. I've replace both shift solinoids, Swapped PCM and all wiring. Have you figured your issue out yet?