For what it's worth - a friend once told me about a rear diff problem on his car. He was also aware of a whining noise at 4th gear, at the same car, while he was having the problem at the rear diff.
To cut a long story short - the diff at his car has been rebuilt, (without touching the gearbox) and suddenly the whining noise in 4th gear has disappeared completely.
The guy came to the conclusion that the whining noise in 4th gear was actually coming from the rear diff, and has been transferred to the gearbox via the drive shaft - simply because the 4th gear has been a "direct drive".
This made me think there might be is a possibility that in some cases, (maybe at R65's as well) there is a possibility that whining noises in 4th gear could be coming from a diff problem.