Audi’s new model assault is to be spearheaded by a fourth A3 derivative that's ready for debut at the 2011 Geneva auto show.This new A3 concept is being classed as "a four-seater notchback" sedan by Audi after it released official sketches of the new car paired with some technical details.Measuring 4.44m bumper-to-bumper, 1.84m across and featuring a 1.39m tall roofline, the A3 concept is around 150mm longer than Audi’s current A3 Sportback. AUDI'S WRX STI KILLER?As a design exercise the A3 concept adds some geometry to Audi’s signature oversized grille (framed by LED-littered headlights), with a compact greenhouse and arching roofline (tracing the A5/7 Sportback silhouette) emphasising its "four-door coupe" proportions. Alongside its hatchback, sportback and cabriolet siblings the A3 concept looks like fine evolutionary styling exercise, indicating that the next generation A3 should be as attractive as the current range of cars. The A3 concept rides on the VW Group’s new MQB platform, designed to accommodate transverse engines and all-wheel drive in a modular fashion for a variety of application sizes – from supermini city cars to MPV's. Unusually for a concept, Audi is quite casual concerning its new "four-door" A3’s drivetrain.Powering the company’s latest show pony is a five-cylinder engine producing 300kW courtesy of turbocharging; odds are that this five-cylinder unit is an evolved version of the engine currently going duty in Audi’s TT-RS. Drive is distributed to all wheels via a seven-speed S-tronic transmission. As concepts go, this is one that could quite possibly give Subaru’s WRX STI project engineering team a world of worry.