CobraDoc,
I am really sorry to tell you the bad news. My guess would be countershaft bearings. I've destroyed and replaced a few.
The reason it doesn't grind in fourth, is that power goes straight through the main gear stack and not thought the countershaft.
My advice is, don't drive it until it's fixed or you may be buying a new counter shaft and the gear cluster, too. Also, I'll bet lots of little ground up pin bearings are floating around in your
oil.
While you have the drain plug out, make sure it has a magnet on it. if it doesn't, epoxy a small rare earth magnet to the concave face of the plug. Let the epoxy set up overnight in a warm place before installing the plug.
I hope this helps and you get it back on the road soon,
Paul