AgencyPower Agency-Power-Blow-Off-Valve The Audi S4 is the sports variant of AUDI AG's compact executive car. The original Audi S4, built from 1991 until 1994, was a performance-oriented version of Audi's 100 saloon/sedan. All subsequent S4s since 1997 have been based on the Audi A4; and as the A4 has evolved from one generation to the next, so has the S4. All S4s have gained a more powerful internal combustion engine, larger uprated brakes, firmer suspension, and larger roadwheels have always been amongst the many upgrades the S4 receives over its mainstream 100 and A4 siblings.[1]