Kb:
Generically speaking (also), there are a number of areas that could contribute to the 4th gear pop-out. Worn pilot bearing, excess end play in the input shaft bearing (wear and/or shimming), wear in the input shaft-to-main shaft roller bearing, worn synchronizer dog teeth on the input shaft, worn synchronizer sliding ring, worn or missing detent balls & springs in synchronizer, excess end play in main shaft, and the above-mentioned worn shift fork.