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.Click here to read more