OK My B16 tranny has been fucked up. Everything was all good until the differential blew up. I replaced it w/ another stock one. So far I have put maybe 800 miles on it, it has made noises, like metal shingling around, now 4th gear is GONE.

I'm going to get a new tranny but why would a gear go out like that?

 

My guess is that one or more pieces of the dead diff (RIP) were still in the case and found fourth (RIP too) to be a nice place to rest and then took a chunck out of it. Could have damaged the synchro, gear itself, or the shift fork. Or you did get all the junk out when you changed the diff and a piece damaged the gear and it just took a while to fail. It sounds like it was noisy immediatly after repair so there is a good chance the gear face was damaged and it just took time to go crunch.

 

****. Looks like new tranny time