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.
Audi Ur-S4 / Ur-S6
The 'S' prefix letter has signified Audi's sporting high-performance production models since the early 1990s. The S4 was the high-performance version of the Audi 100 saloon, and was known originally as the Audi 100 S4. It featured a 20-valve, 5-cylinder turbocharged engine and quattro permanent four-wheel drive, providing outstanding traction and roadholding in all weathers. Befitting its status at the top of the Audi 100 series, it was luxuriously appointed and featured as standard a lot of equipment that was optional on lesser models. The car was available in saloon/sedan and Avant (estate/wagon) versions.