2013 Mitsubishi Canter 6500kg GVM 5 speed tip truck. Recently had a new clutch installed. Drives fine until the engine warms up. Then will not go into 4th gear. All other gears work fine.
The external shift linkage probably had to be removed and reinstalled when the clutch was changed, and may have been re-installed or re-adjusted incorrectly. If that isn't the problem, then there's an internal fault in the gearbox.
The linkage is probably easier to get to, and — if the gearbox worked correctly before the clutch R&R was done — more likely the cause of the problem. I don't think the problem is the clutch itself, as the gearbox can be shifted and the other gears work correctly.
Have a competent mechanic inspect/adjust/repair the linkage. If that doesn't do it, the gearbox will have to be opened up.