4th gear problem

I can tell you exacticly. Never lift!
Ok but; the clutch teeth on the front of the gear are chipped off, and so are the matching teeth on the slider. I used to fix these all the time. Hundred and hundred of 'em.
Guys sometimes think it's the struts, but it's not.The struts only have one job; to carry the brass into the brake-cones.
There is a good chance this started with worn shift forks not driving the slider far enough, or a sloppy bearing; but most likely abuse; whether from ignorance or somebody just didn't care, I cannot say.
I doubt there are any shops that will fix those gears for you tho, and that's a shame, cuz it's pretty easy.
Jumping outta 4th is unusual tho, but I'm surprised it is cuz that particular gear is way out there and hard for the slider to reach.
The gear is super easy to fix, but you will probably have to replace the synchronizer or at least the slider. A full rebuild is,of course,recommended; but mine has gone for many thousands of miles on all used parts including gaskets and seals. I just love that dinosaur.