ET35 i wouldnt go any less5 x 112 PCD with and (offset) ET 42 or 43 (although a few either side will be ok).
ET35 i wouldnt go any less
i have ET30 rear and 35 front sits just nice
thats with 8" rims.
Sorry accelerated wear on bearings were talking 10mm here not 30+mm and im fairly low on the rear with ET30 and never have rubbing even loaded .Max width without lowering is prob 30 on the rear and 35 on the front. With lowering it'd prob make those numbers go up a few mm. Also remember the lower the ET, the more wear you're putting on the bearings - so be prepared for accelerated wear.
Sorry accelerated wear on bearings were talking 10mm here not 30+mm and im fairly low on the rear with ET30 and never have rubbing even loaded .
ET30 = 15mm wider than the S4....
That said, I'd be tempted to take mine out from ET45 to ET35 or so
edit: any idea if having a wider track at the rear will mess with the Quattro at all?
of course thats what they model them as DOH.