Yeh as above, the PFL S4 had big fronts, but the same 245mm rears as the rest of the range.
FL S4's got 256mm vented rears.
A cheaper/more available upgrade would be to use the rear setup from a B7 A4, some models (not sure which, but quite a few AFAIK) got a 280mm rear disk, which i'm fairly sure should bolt right up to the B5 quattro hub.