The transmission will not go into 4th gear at all now that it's cold. When weather was warmer, it would shift after 15 miles of 60mph driving. The relay does work and the lower 3 gears shift ok. I changed the tranmission fluid. Any ideas?

We had this one. It could be wiring, but the warm-up behaviour suggests that the small passage in the OD solenoid on the side of the transmission is plugging up.

You might try a transmission flush and refill, especially if you haven't been doing it regularly in the past. If this doesn't work, you'll have to have the solenoid removed (need a hoist) and the passages blown clear. I think the mechanic will need to energize the solenoid to open the passage for this. It worked for me, but he did warn that if he broke a bolt, he would have to drop the transmission to finish the job properly. He didn't, but he must be speaking from some experience!