I think I found my problem. My transmission shifts fine when the transmission fluid is cool, but when it gets hot it is no longer holding the necessary pressure to keep it in 4th gear. This happens because the fluid thins as it gets hotter. As this occurs the fluid is slipping by older worn seals and releasing pressure, causing the downshifting. Bottomline is that I need a rebuild. Just wanted to lay this one to rest and update all of you. I hope this may help others having similar problems.