When I'm getting on the highway, at about 3 1/2- 4k. Shifting into 4th is really rough. And before letting the clutch out, it seems to make a short gring sound. The car is going in the shop for it's 6k service very soon. I just wanted some ideas before it goes. Sorry I just realized I should have put this in the tech forum!!!!!!!!!!

 

It has to do with the synchros. Mine does it too. Other than that i dont know. transmitions are not my thing.

 

lathe shaft bearing is on its way out or its the clutch release bearing,also the the selector gator,something could well be jamming it,a mate of mine had the same problem with his,turned out to be a worn selector...easy cheap fix...

 

just had my 4th and 3rd gear synchros replaced on my 05 STi. The car only had 17k miles on it so it was covered under warranty. Capitol Subaru did an excellent job by the way. I was told that the 04 -05 are manufactured with weak Synchros that are urethane coated as opposed to 06 and up which are carbon coated. Apparently, subaru recognized this as a problem thus changing them in later models.

This is a common issue. Just go to google and type in 4th gear grind WRX. Good luck!