z31 manual transmission wont go into 4th gear

[mattjrbdet]

I have an 86 n/a z31 that wont go into 4th gear at all when the car is off or on. All other gears worked fine until last night when i drove it for about an hour on the high way. I had to down shift into 3rd and it went in kinda rough. After that it wouldnt go into 3rd either until I shut the car off, manually shifted through the gears and then it drove fine again (minus 4th gear) Point being, do you think its a shift fork, 3/4 synchronizer assembly, or a main shaft problem? I originally thought main shaft because i know 4th is 1:1 and all other gears worked fine, but after 3rd woudlnt go in Im debating on weather or not a synchro assembly could do that. I have experience rebuilding manual and automatic transmissions, and access to a press so I am able to replace the 3/4 synchro.

Just looking for opinions before I drop the transmission and pull it all apart, or buy a new transmission and throw it on.