There is awesomeness to the prefix "twin-turbo." It could be a Briggs & Stratton Vanguard engine (awesome in its own right) that you're talking about, but prepending any engine designation with the modifier "twin-turbo" can suck all of the air out of a room. Owners of lesser cars just nod in acknowledgment of their inferiority. Audi fans have lamented the evolution of the S4 from a twin-turbocharged V6 in the B5 version to V8 power for the two most recent iterations. Many feel that the V8 powerplant doesn't have the addictive frenetic nature of the old S4. For the next generation, Audi is returning the S4 to its roots as a more refined version of what you'll find in the Evo or the STi. The B8 S4 has reportedly been lapping the Nurburgring with a force-fed six-shooter underhood.
With the A5/S5 joining the lineup, the S4 doesn't have to worry as much about serving customers who'd rather have a GT than something more serious. Thus, an artificially-aspirated V6 displacing three liters and employing VW/Audi Group's FSI direct-injection technology and Audi's Valvelift actuation system can be parked in the S4 without fears that the hardcore nature of such a powerplant would be off-putting. Whether the external compressors are turbos or a supercharger is the subject of rumor. Also open to speculation is the horsepower output, but it'd be safe to bet that something slightly higher than the 300 HP packed into the BMW 335 can be expected. Also expect the S4 to employ Audi's Sport Differential to offer tweakable chassis attitude, as well as the obligatory styling tweaks and sumptuous cabin, too. Click the Read link below to view a couple renderings of what the next S4 might look like courtesy of Fourtitude.
[Source: Fourtitude, Rendering: Fourtitude]