Can't give you any details about a lab in the area unfortunately but, If you dont' get any other responses and if John, the owner of the Camera Exhange on N. Palm Canyon drive didn't close his business yet (Posted in another tread recently) you can check with him, i'm sure he would tell you where 4x5's are processed in the area.
I don't remember the address but it's about a block north of where N. Palm Cyn Dr. turns into a one way. (The Drag)
Enjoy the Desert..
jan