No matter what (upshifting, downshifting, cold or hot), when switching into 4th gear, there is a grinding sound before it shifts in. 1st, 2nd, 3rd, 5th and 6th gears work just fine. I would like to know if this could be more of a clutch or gearbox related issue. Thanks in advance.
As @paulster2 said, your 3-4 synchronizer is worn out. There is a synchronizer between each set of gears, i.e., 1-2 and 2-3, etc. The synchronizer is like a little friction clutch that encourages pairs of gears to turn at the same speed before they mesh so that they don't grind. Your transmission can be rebuilt. It's probably best to replace all the synchronizers at once because most of the expense is labor.
Meanwhile, to prevent grinding, you can double-clutch for the shift from 3-4. It works like this:
- Depress the clutch as you release pressure on the gas pedal
- Shift to neutral then release the clutch
- As the engine RPM slows down to where it will be in 4th gear, depress the clutch as you shift into 4th
- Release the clutch as you step on the gas to resume acceleration
This takes only a moment. It takes longer to describe than to do it. If it still grinds, you're doing it wrong. You need to practice and set up a rhythm until it becomes automatic. Big rig drivers whose transmissions don't have synchronizers shift this way without grinding. Your grandfather shifted this way before synchronizers were commonplace on cars.
The most important timing step is to shift from neutral exactly when the engine speed has fallen to just the right RPM for your current road speed in the gear you want.