Steve,
I'm 6'3" and 15 stone, so in no means small; therefore, I should feel any issue with the seat, that would compromise comfort. I can honestly say that I have never noticed what you've photographed there.
I wonder if this is one of the cars that they had changed the padding on. There were a batch of initial S4 cars, that received a slightly different thickness of padding on the seat base (mainly at the front of the seat - not the rear where you have photographed). It actually raised the seat height and meant taller drivers found it hard to get low enough (in my case it meant my knees hit the wheel and my head sat proud of the windscreen). MMC soon reverted back to the standard padding.
My other guess would be that the seat padding has slipped or moved. I don't see it as being an issue to fix and nor, as mentioned above, have I ever felt the seats to be uncomfortable. In fact, I find the seats very nice for long journeys. My car is currently being serviced, so I can't check it and photograph the seats for you; however, perhaps someone else will oblige.
Cheers
Simon
