Audi S4
Audi's quattro four-wheel-drive system was standard on the C4 S4/S6; a first-generation Torsen-based system which used the "T-1" automatically biasing centre differential (ATB) was used on vehicles equipped with manual transmissions, while a transfer box based system was used on vehicles equipped with an automatic transmission.
Quattro (four-wheel-drive system)
Starting from 1988 on older generation Audi 100 C3 platform and Audi Quattro until the end of their production, and on new generation B3 platform (1989–1992) Audi 80/90 quattro, B4 platform (1992–1995) Audi 80, Audi S2, Audi RS2 Avant, C4 platform (1991–1994) Audi 100 quattro, Audi S4, later C4 platform (1994-1997) Audi A6/S6.