Ive been running Mk5 R32 front callipers and carriers for a while now on my B6 A4 using S4 discs and pads The callipers and carriers are the same as the B6 S4, B7 S4, S3 8P, A3 3.2. The discs are the same size, 345mm, but the offset on the VW disc is different by about 3mm and the spigot hole is a lot smaller so wont fit on the A4. Dont know if the A3 discs will fit the A4 though.
Hope this helps.