Are you certain that it's the gear box and not the final drive? If you're certain it's the gear box, then that means it could be one of 7-8 bearings within the gearbox itself. Obviously, you'd want to renew all bearings and synchronizers if it has to be opened up. However, since you mention it gets noisy in 4th gear that tells me that one of the 4 carrier bearings (two for main shaft and two for countershaft) is the culprit.
If you're hearing bearing noise, then it's only a matter of time before a bearing fails entirely. It could be tomorrow, or it could be 5 years from now. There is no accurate way to gauge when it'll fail.....but know that it will. And yes, they wear from wear and old age, but wear is also hastened from oil starvation, not changing the oil regularly, using gear oil instead of automatic transmission oil, misuse, or any combination of the above. So at some point in the near future a thorough going through of the gear box will be necessary.
If you aren't able to do the work yourself make sure that you find someone capable. Various tolerances must be adhered to. Namely, end float of the main shaft and counter shaft (this assures proper meshing of the gear teeth and quietness). The only specialty tool required is a slotted nut socket to remove the rear yoke and one of the gears on the mainshaft. There is no alternative to these tools unless you make your own. Again, make sure that whoever does the job is knowledgeable about these gear boxes, has the tools, and has the ability to understand how and why end float is necessary to set correctly.
