Hi Jase,
I have exactly the same problem with mine (r32 GTR). 4th crunches only when I am giving it heavy throttle, but not full thrashing. I have a feeling its from flat changing by the previous owner. My suggestion is when going from third to forth > engage clutch then move to nuetral > release clutch > engage clutch again and then put into fourth. This should take all the load of the box and make a smooth change. Option 2 is to rebuild and to shift a bit slower in future.