850 Commando Jumping Out Of Gear

... from 4th would be very odd as the camplate would have to rotate about 270 degrees and go through 3rd and 2nd on the way. Jumping out is usually caused by wear on the dogs (in this case the ones that lock the sleeve gear to the mainshaft). But I'd also be looking to see whether the selector fork is bent or badly worn, and whether you have full travel on the quadrant.

Many people would consider that 50 is quite slow for 4th gear......

There are "false" neutrals between 4th and 3rd and between 3rd and 2nd. As Ian said wear of the cogs is the most likely cause. The cogs are undercut so they shall lock together. It seldom happens that the spring in the pawlcarrier is faulty. Had one a 1/4" too short two weeks ago. The idea of the pawl is to keep camplate in distinctive positions. Thus minimize selector forks wear. A bit like belt and braces. It is also meant to keep the box in gear when you go from acceleration to engine braking. I.e. changing from one side of the cogs to the other.

Edit: Before the time when gearbox manufacturers started to undercut the cogs, keeping gears positive locked together, gears could disengage more easily.