@Jag was pretty close....3 rebuilds in 2yrs all different causes (the second time was parts failure due in the clutch PROBABLY caused by over-torquing a bolt so a human error not machine failure)
Yes you have to pull the engine for trans repairs. I save at least AUD 2000 by doing the work myself. It would cost around AUD 4000 to rebuild the complete trans using OEM spares.
R3's are very easy to work on - I'm not a mechanic but i can use a torque wrench and follow a workshop manual so the repairs weren't difficult,just took my time.
Search gearbox and trans issues and you'll find several threads that will help you understand your current 4th gear symptoms a bit better.