Unlikely to be universal joints if it's only doing it in fourth gear James....
Does it not do it in the other gears at all - ie when setting off in first / reverse ?
Mine clunked like hell when the propshaft UJ's went, but it also clunked like hell when the rear Diff went shall we say, over tolerance for backlash...
It also clunked like hell in third gear which was knackered !!
I decided that driving it wasn't safe until I found out what it was, and if it's that bad, I wouldn't drive it....
It isn't necessarily dangerous, it's just that when whatever is clunking lets go completely, it will take a lot of expensive components with it....
Lie underneath it, grab the propshafts (both shafts at both ends) and firstly waggle them up and down whilst watching the UJ's, this will tell you if it's UJ's , which isn't that expensive (although neither are new propshafts which is a lot less grief
!!!)
Next, grab the props and twist them to assess how much play you have in your Diffs. My props were turning almost 180 degrees without the crownwheels connecting with the pinions (still working, very noisy though
!)
If all that is good, you have a gearbox problem my friend...........
Hope that helps
Seeya
Martin