The Audi S4 is the high performance variant of Audi's compact executive car A4. 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.

A more powerful internal combustion engine, larger upgraded brakes, firmer suspension, larger roadwheels, and distinctive sheetmetal,^{[citation needed]} styling clues and badging have always been amongst the many upgrades the S4 receives over its mainstream 100 and A4 siblings. In markets where the even higher-performance Audi RS4 is not offered, the S4 is the top-of-the-line trim of the A4 family.

Like its regular A4 counterpart, all S4 variants have had longitudinally oriented, front-mounted engines. A single turbocharged 2.2 litre inline five-cylinder powered the original C4 version, and a 2.7 litre twinturbocharged V6 engine was found in the B5 generation. The B6 and B7 versions shared a common 4.2 litre V8 engine, the first time that a V8 engine was placed in a compact executive car, placing it in direct competition with the BMW M3 which at the time had a 3.2 L inline 6. The recently introduced B8 generation uses a supercharged 3.0 litre V6 TFSI engine and competes with the BMW 335i.