JB,
There's a good chance that you rounded the dogs on 4th gear. The "fingers" that cause one gear to lock into place can get worn and cause the very problem you're dealing with.
You may not find any obviuose damage until you pull hte tranny and unspect the edges of the dogs.
Good luck,
Manny
p.s....at least you didn't have a gear sieze "onto" the gear shaft. I just did and you can't believe the parts list! You're probably looking at a gasket set and a gear or two.