If it has been grinding for awhile usually the gear and sleeve is worn/chewed up and thats where it gets expensive and not worth while because alot of the parts go to service files which include a bunch of other parts, 4th gear is the one thats get the abuse because it is actually part of the input shaft, you wont know untill you tear it apart. If the "house" looking teeth on the gear or sleeve are torn up from grinding it will make it hard to shift if you fix the spring inserts.
If you have a spare trans or 2 laying around you can usually have what you need. The spring inserts in 3-4 can be done without taking the gear set apart and they are only a few bucks so its worth a try sometimes..
