Yep, seen it many times (and not just 4th gear either) - there are a few freaky things that can cause it, but most of the time it is caused by a worn gearbox.
Is this something that has just started happening, or has it been doing this now and again, but getting worse?
If it has just started happening it may be worth trying a few things before you condemn the gearbox.
R 🙂
40hp, 6V and ...er... slowly making improvements
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