Bad luck. I think its safe to rule out defective covers with every one of your replacement clutch covers. to be unlucky once is enough, twice is remote, 3 times and should go and buy a lotto ticket.
The problem with jobs like these is that the work is largely hidden once the gearbox is refitted so youre in the lap of the gods.
I wonder if the cover plate was torqued down in the proper sequence or did they just air gun one bolt in then the next one to it rather than slowly doing each one up diagonally opposed? Ive heard of the poor fitting like this damaging the springs on the clutch friction plate.......unless the flywheel is excessively worn I cant see that causing a problem with the cover plate.
When they have changed all these clutch covers have they inspected them to see if there is any obvious damage and what might cause it? Swapping it for a new is all well and good but youll never get to the root of the problem.
I wonder if the release bearing fork bushes are worn? It can be many things really - can you give us a few more symptoms and how it drove, how it failed, noises etc.
be thankful its under warranty cos after 4 clutches youd be needing a small mortgage to pay for the parts labour and VAT!