I had something similar on my JTD about 2 years ago. Didnt like 4th at all and was sticky going in and sometimes didnt wanna go in at all.
What i did was to change the oil and put some good castrol stuff in. I also put 2 gearbox oil additives in aswell which is thicker than the oil.
This sorted it and it has been ok since - touch wood.
Definately change the oil and you may find than some metal bits/shavings may come out. If i remeber correctly i think it is the syncromesh for 4th gear which basically disintergrates and messes evrything up!
Stu