Guess I'll update on this thread anyway.
Engine out, old gearbox pulled, inspection plate off nothing blatantly obvious but turning the input shaft by hand and shifting gears reveals that even though the fork pushes onto 4th gear, nothing engages, input shaft spinning merrily, output nada. If I spin the output shaft the gears move up until the point where 4th is. Once it did grab and 4th worked (in fact, during the last drive I got in 4th twice but later could not again). Difference between off the engine on is before, when on I couldn't even get the stick down where 4th should be, now off I can get the stick there but nothing catches (9 times out of 10).
Small piece of metal shaving sticking out from under the brass slider fork thing. No other chunks in the case but there was another when I drained it before the pull. So not sure what's up with it, doesn't matter, replacement box on it's way in.
Just trying to decide how many other things to disturb. Release bearing, clutch plate, pressure plate......don't know. Probably down right silly not to keep digging and at least inspect the clutch plate, no?