Bugatti’s worst kept secret, a forthcoming four-door supercar, was shown to current Veyron owners at the company’s Molsheim headquarters over the weekend.Although currently only a concept, Galibier – named after the notorious Col du Galibier, a Swiss Tour de France Alpine pass - has the makings of a phenomenal four-door performance car.No turbo?Powered Galibier is the Veyron’s outrageously powerful 8l W16 engine, mounted above the front axle, instead of amidships.A mechanical running change for the 8l W16 is in terms of its forced induction regime, with dual superchargers replacing the quad-turbocharging configuration seen in the current Veyron. Expect power top peak just shy of 600kW.Galibier's drive is to all four wheels, though we suspect the Veyron's bespoke seven-speed dual-clutch transmission will be supplanted by a planetary geared self-shifter for this four-door application.Bugatti has enabled the engine to run on ethanol too, although we doubt this was done to really placate environmentalists – as Koenigsegg has shown, ethanol is one way to make a supercar even more powerful.Guesstimates credit Galibier with a 350km/h topspeed, down significantly on the Veyron, yet still otherworldly compared to anything else.Classic Bugatti styling cuesThe concept’s styling has obviously been influenced heavily by the car’s monumental topspeed potential (with surfacing pandering to aerodynamic considerations) and Bugatti’s aesthetic heritage.Harking back to the company's classic Type 35 and 57, Galibier features a signature centre spine on the roofline. The dual-tone surfacing (compromising polished aluminium and carbon-fibre) folds to a rump design very reminiscent of the EB four-door concepts VW commissioned in the late 1990s.Galibier’s split rear window is pure Bugatti – though we doubt a production version, expected around 2012, would feature this styling detail. In concept form the cabin is starkly minimalist, seating four in Porsche Panamera/BMW X6 fashion. Housed in the elaborately surfaced timber dash are two centrally positioned gauges indicating road-speed and available engine power.Galibier’s most notable interior trinket is the Reverso Tourbillon clock in the dash, crafted by the Swiss watchmaker Parmigiani, which can be removed and worn as a wristwatch if you tire of your Patek Philippe timepiece...The Galibier is expected to be officially unveiled, with further technical details, at the Frankfurt motorshow later today.