As Rud said its the disc and pads on most cars...
Not to meation the size of the 8 pots on the RS4's wouldnt fit under most wheels without spacers and of coarse bigger radius wheels, then you have to think about the Disc size increase as you wouldnt want the calipers to miss the disc because the disc is too small for the pad size on the RS4 calipers..
Then you have the next
problem master cylinder, if its not up to the size, your end up with spongey brakes... as i think your find the RS4 uses at least 4ml bigger cylinder to get the 8 pot pairs at the front and the 4 pot rears moving..
I only know this as i did abit of reading into the calipers in the VAG range as I'm upgrading my brakes to Porka GT3 calipers and R32 discs.. and I'm JUST fitting them under my 19" wheels..