I have a 3.0TDI S-Line Quattro (2006 - latest shape) and the car has the 18" RS6 style 5-spoke original S-Line alloys.
The offset on these wheels is ET43, and they are pretty close to the edge of the arches as standard. As the RS4 offset is ET29 (i.e. 14mm further-out) then you'll end up with the car looking silly, like some kind of american hot-rod!! I would stick with an offset of 35mm or more. Remember, the lower the "ET" number, the more the wheels stick out from the hub.
Incidentally, I have a B7 A4 Cabrio, which had the 18" RS4 style wheels on that as standard. When I changed them for 19"s a few weeks ago, I tried the 18" RS4 style wheels on the A6. For some reason, they looked really small, much smaller than the same size standard wheels. Not sure if it is just the design, but they didn't look right. I know you're looking at 19s, but just a word or warning!!
Cheers