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In neutral the only two bearing that are likely to make a noise are either; the release bearing. On a 120 this does not run in contact with the cover plate, unlike many modern cars. The adjustment is done by feel, you should be able to pull the release arm backwards by about 3mm or so bringing the bearing into contact with the cover plate. As the clutch wears it will gradually eat up the free play necessitating adjustment from time to time.
Or in neutral the input shaft can also turn so it is just conceivable that you can hear the bearing on the input shaft. In neutral, or in gear the pilot bearing doesn't do anything other than stop the input shaft wobbling about since the clutch locks the flywheel and input shaft together, it only functions as a bearing when the clutch is released during gear changes. Normally bad pilot bearing make it hard to select first gear from rest, if they drag the input shaft can keep turnig despite the clutch being released.
4th gear is the only direct gear, therefore if the noise is only in 4th it means it is nothing to do with the gear cluster bearings in th bottom of the 'box. It is either the input shaft bearing in the front of the box, or the mainshaft bearing in the back of the box.
Personally I'd first check the clutch adjustment and change the gearbox oil, ordinary 20-50 engine oil is fine. The drain plug is magnetic so if something major is failing bits of bearing can be stuck to the plug, bits of swarf are normal. Even horribly noisy these boxes run and run.
Regards
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