well thats informative....
anyway, same thing happened to me about 400 miles on fresh t5 rebuild, got stuck in fourth and then when i tried to get it out, felt like it all just came apart inside, would kind of go into neutral but no other gear, and when i tried to go from "neutral" to fourth it felt like there was now synchro
if this is similar to your case, you need to tear apart your tranny and rebuild it. my problem turned out to be that one of the three things the sliders slide back and forth on (lever-type things) broke and got jammed between slider and shaft, so good luck to you (don't go hard on aftermarket tranny rebuild kit)