Couple things to check Tim. First check the adjustment on your shifter to be sure that fourth gear is engaging properly. If that seems ok then I would say that the synchro is worn and is not holding on anymore. Another possibility is the shift fork is severly worn down and is not pushing the slider up far enough. Often the shift fork will wear down when someone is driving with their hand resting on the shifter while it is in fourth gear. This can cause the fork to ride on the slider and wear out both the slider and the fork. Sounds like it may be time for a freshen up.