The third generation, B6 platform S4 debuted in mid-2003. Despite having moved to yet another entirely new platform, the S4's most publicized new feature was its engine. It became available in three bodystyles for the 2004 model year: the S4 sedan, the S4 Avant wagon and a convertible, the Audi S4 Cabriolet.
Replacing the 2.7 L bi-turbo V6 engine of the previous generation S4 was a 4.2 L, 40-valve V8 based on the unit from the A8 and producing 344 PS (253 kW/340 hp), making it more potent than either the S6 Plus or RS2 and nearly as powerful as the B5 platform RS4.
With such abundant power on tap, the B6 S4 could accelerate quickly enough to meet or beat the times of most sports cars. Road tests conducted by American automotive magazines indicated that it could go from a standstill to 100 km/h in 5.2 seconds, and make it to 200 km/h (124 mph) in slightly over 20 seconds.
Like its predecessors, the model was standard with a six-speed manual and Quattro, the Tiptronic transmission remained an option. The B6 platform S4 was replaced by the B7 platform S4 in late-2005.