Aye, the front wheels are still gleaming, whilst the rears are minging already
TT - does that mean it's £4k+ for TWO ceramics 
and the rear discs are still iron?
Yup!!
But the ceramics are claimed to last any where between five to ten times longer than the conventional setup.
Now, on the standard RS4 front brakes, a pad set will cost about £250-280, discs (will probably need changing every pad change, ever other at best) are around £300-400 EACH - so you could be looking at £900 for a disc and pad change. Some peeps have reported needing new (conventional) discs and pads after around 6,000-7,000 miles. That is similar to what I was getting on my last S4.
Looking at the Porsche ceramics vs conventional iron setups, the extended life of the ceramics does seem to be standing up to the manufacturers claims (providing they are not abused).
And then there is quantum leap improvement in braking performance!
