Wont select 4th

Hi guys, my gearbox wont select 4th even when the engine is switched off. The problem started when I went up a steep hill last year. After that it went, but eventually came back intermittently. it hasnt come back for a while now. it seemed as if it was a bushing on the top of the box that was gone (the previous owner placed a piece of hose to compensate). I got a genuine bushing and put it in, but the problems still not solved. My mechanic tells me that if he pulls down the box, I might have to replace it, as the parts for the box are not readilly available. Anybody have a possible solution to this problem. Its an 89 gq wagon.