Just thought I would post this up as its a common and extremely annoying problem with the Mk IV. In my experience its not the seat that actually creaks - its the brackets the seat sits on. The offending brackets are at the rear of the seat and can easily be accessed without any need for removal of the seat.
Just been out today and solved it easily with a can of WD40. Few shorts bursts of it in to the mounting brackets and creaking has disappeared completely.
This is the second Mk IV I've had where the WD40 solved the problem of creaking on the drivers seat. Simply direct the spray into the brackets that connect the seats to the runners and jobs a good un'.