RS is Audi's ultimate prefix, surpassing the S models and denoting maximum performance. U.S. buyers have only seen one RS model: the 2003 RS6, a wild-child A6 rumbling with a blown, 450-hp V-8. But the RS6 was actually the third Audi RS, following the 1994-95 RS2 (based on the Audi 80 wagon) and the 2000-01 RS4, a 375-hp, twin-turbocharged screamer fashioned from the first-generation Audi A4 Avant. A move away from turbocharging is a big departure. The RS4's new naturally aspirated, direct-injection V-8 is ultraresponsive and addictively light-footed in the way it summons more power and whips up more torque. With 420 hp, the 2006 RS4 will accelerate from 0 to 62 mph in a stellar 4.8 seconds. Whereas the most recent RS, the RS6, had a five-speed automatic, the new RS4 is fitted with a six-speed stick shift. The shift linkage is quick and slick, and clutch action is light and progressive. The transmission sends torque to all four wheels, but the rears get priority. By altering the Quattro drivetrain from a 50/50 to a 40/60 front/rear torque split for use in the RS4, Audi wants to zoom in on BMW and Mercedes-Benz. Now that the rear wheels get a bigger share of the action, the front wheels can concentrate on turn-in and braking. This results in light and linear steering, though it's still not as telepathic and communicative as those fitted to the best rear-wheel-drive competitors. Borrowed from the Lamborghini Gallardo, the huge brake rotors (14.7 inches front, 12.8 inches rear) kill energy with the finiteness of a master switch, again and again, with great determination and only gradually showing faint traces of fade. In the wet, the discs are intermittently swept dry to improve response and stopping power. Americans may be unfamiliar with the RS4, but when it finally arrives stateside this spring, they're unlikely to mistake it for a generic A4. The go-faster Audi boasts bloated and Botoxed bumpers front and rear, plenty of brightwork, additional air intakes, f