Audi's A4 is a svelte and sophisticated sports sedan with power and handling to make quick work of most twisty roads. But for some die-hard performance enthusiasts, the basic version is not enough. Enter the S4, a bulked-up bruiser equipped to take on all comers. Armed with a twin-turbo version of the 2.8-liter V6 used in the larger A6, plus a stiffer suspension and bigger brakes, the S4 is a world-class athlete. Yet true to its German roots, styling cues are subtle, meaning this sedan can go about its business quietly until its driver is ready to rumble.