B6 S4 I believe fit right in place and the beauty with the B6 S4 discs is there are 3rd party manufacturers of the discs from the likes of EBC, Forrodo, merlin and so on. So cheap as chips.
I hope someone can verify that they do fit straight in.**** EDIT *****
Whoops hang on.... I think I meant it happily replaces B5 discs.