Naturally, four-wheel drive is standard
Quattro is standard on the S4, and so is the 40:60 front to rear torque split. But to get the best out of four-wheel drive, you should specify the optional sport differential. Similar in function to BMW's automatic performance control device available on the X5 and the X6, the trick diff distributes torque between the rear wheels in a progressively variable fashion. Unlike the BMW concept, the Audi hardware also works under trailing throttle and even when the gearbox is in neutral. As the vehicle turns in, the sport diff automatically diverts most of the propulsion forces to the outer rear wheel. This reduces understeer, allows you to work with a more moderate steering angle, and improves the roadholding as well as the directional stabilty. At the limit of adhesion, when the rear ends begs to be reined in by the ESP, the extra set of cogworks keeps the tail in line by feeding twist action to the wheel that is closest to the apex. The response time of less than 100 milliseconds even beats the ESP black box by a small margin. The maximum torque difference between the rear wheels is substantial, but the biggest advantage of the sport differential is that instead of eliminating excess energy, it cleverly and smoothly redistributes it.
Audi Drive Select: fancy name for fancy dynamics
To specify the sport diff, you must first opt for Audi Drive Select. And if you're doing that, you might as well also go for the extra-cost adjustable dampers and for the so-called Dynamic Steering. Drive Select offers an easily accessible personal choice of three different tuning stages, labeled comfort, normal, and dynamic. Via the MMI controller, you can dial in the preferred calibration of engine (throttle response), automatic transmission (shift pattern), dampers, steering, and sport differential. In a car like the S4, dynamic provides a nice mix of sharpness and balance, of tactility and feedback, of intuition and agility. It's still possible to deactivate ASR and then ASR plus ESP, but this move doesn't yield much anymore, because the north-south and east-west torque flow is managed to perfection by a couple of gear sets and their electronic brains. As a result, hard cornering is no longer a mix of more or less understeer. Instead, the nose turns in, the rear end tracks to match, and the ensuing four-wheel drift is easily modulated by throttle and steering in much smaller nuances than before.