I hate T5s (sorry for the ranting)

[richd]

My bet is that you're not rebuilding them correctly. Try this (with a buddy):

1. Remove driveshaft.
2. Have your buddy push the end of the mainshaft towards the engine as hard as he can. It's easier to do this from the input shaft, but I'm assuming the trans is still in the car.
3. While he's pushing, try to get the trans into neutral.

If this "unsticks" 4th gear, you most likely have either not shimmed the input shaft at all, or you did not shim it enough. I bet you have way too little preload (read: too much fore/aft play) on the input shaft. While your in there, check the coutershaft endplay also.

I think the endplay should be 0.002 to 0.005, but I don't remember exactly. Also, don't forget that the trans will loosen a little as you drive it more, which may be why this didn't happen right away.

Let us know how it turns out...

Good luck,

Rich D.