Appearing this fall as a 2010 model, a new breed of Porsche – like no other Porsche ever built – will begin roaming the highways. The four-door Porsche sedan featuring an odd-but-useful hatchback body style, seating four and offering more luxury amenities than any previous model in its history will be called the Panamera.

Porsche began redefining its brand when it introduced the Cayenne for the 2003 model year. An SUV didn’t exactly fit into Porsche’s performance image, but the Cayenne performed both on the road and in dealer showrooms. The Cayenne doubled Porsche’s sales to roughly 100,000 units worldwide, allowing the company to grow its dealer network and move into new foreign markets. With the introduction of the Panamera, Porsche intends to add another 20,000 annual sales and fill a gap in the grand touring luxury performance market where Porsche has never played.

The Panamera will join the 911, Boxster/Cayman and Cayenne as a fourth model lineup for Porsche. It will compete against the likes of the Maserati Quattroporte, Mercedes-Benz CLS and S Class, BMW M5 and Audi RS6. All of those vehicles have useful rear seats, and so does the Panamera. In fact, the roomy rear seat is partially responsible for the way the car looks. Porsche says the design contains elements of a coupe, a sedan and a wagon. Porsche wanted a coupelike profile for a sporty look, the backseat room of a sedan and the cargo utility of a wagon.

The Panamera will join the 911, Boxster/Cayman and Cayenne as a fourth model lineup for Porsche. It will compete against the likes of the Maserati Quattroporte, Mercedes-Benz CLS and S Class, BMW M5 and Audi RS6. All of those vehicles have useful rear seats, and so does the Panamera. In fact, the roomy rear seat is partially responsible for the way the car looks. Porsche says the design contains elements of a coupe, a sedan and a wagon. Porsche wanted a coupelike profile for a sporty look, the backseat room of a sedan and the cargo utility of a wagon.

Those parameters led Porsche design chief Michael Mauer to choose a hatchback design instead of a traditional sedan body style. The hatchback allowed for the rear headroom Porsche desired, as well as the rear cargo utility and the sporty coupe rear profile.

The visual message, the proportion, should say I’m more of a coupe than a traditional sedan or wagon. We knew from the very beginning that a hatchback was the right way. Who needs just another three-box, conventional, boring sedan design?” Mauer asked.

Mauer believed he couldn’t deviate too far from traditional Porsche design. “The Panamera is pointing out a new direction, but we are also staying with our design language,” he said.

The 911 influences are unmistakable, but the rear end had to be new. Porsche kept the company’s signature rear shoulders, but extended the roof line to the end of the car. Hidden at the back of the hatch is an active rear spoiler. It pops up at speed to increase rear downforce.

The 2010 Panamera will come in one rear-wheel-drive model called S and two all-wheel-drive models called 4S and Turbo. The S and 4S models will come with a 400-horsepower 4.8-liter V-8, which will launch the 4S model to 60 mph in just 4.8 seconds. The Turbo will get a twin-turbocharged version of the same engine making 500 horsepower and rocketing the car to 60 clicks in 4.0 seconds. Both S models will top out at 175 mph, while the Turbo will reach 188 mph. Just don’t try this at home.

Both engines will use Porsche’s seven-speed Porsche Doppelkupplung (PDK) automated manual transmission. The PDK uses two clutches, one to hold the current gear and one to ready the next. The Panamera also will come with a start/stop feature that turns the engine off at stoplights to conserve fuel.

Underpinning the car will be an all-new architecture developed specifically for the Panamera. Porsche says the engine has been pushed as far back as possible to optimize handling and achieve a 52/48 weight balance. The structure will employ a lightweight monocoque construction using steel, aluminum and magnesium.

The suspension will feature a lot of technology. The independent front and multilink independent rear suspensions will come standard with the Porsche active suspension management system, which electronically controls the shocks for three levels of ride quality. Standard on the Turbo, and optional otherwise, will be an adaptive air suspension that can tune the ride/handling balance even further. Also optional will be Porsche’s dynamic chassis control, which firms up to limit body roll in turns and disconnects the anti-roll bars to improve straight-line comfort on bumpy roads.

Porsche says 911 owners will feel right at home in the Panamera because the low driving position provides a similar connection with the road and the interior touch points are comparable. However, the company also has pushed the envelope to outfit the car with new levels of luxury. Leather upholstery will be standard and Porsche will offer 13 color-and-material combinations, including Carbon, Tineo and ash wood. A navigation system will be standard, and Porsche will also offer a high-end Burmeister sound system with 16 speakers, a 300-watt subwoofer and 1,000 watts of overall power.

A distinctive center console will run the length of the interior. In back, it will bisect two individually sculpted seats Porsche designed for comfort. The rear hatch will open to reveal a cargo area with 15.6 cubic feet of space, and the rear seatbacks will fold forward to increase cargo capacity to 44.2 cubic feet.

While Porsche’s first-ever four-door sedan promises to offer interior luxury and cargo utility, it most certainly won’t sacrifice sporty handling. And in that way, this unique Porsche will be like so many legendary Porsches before it. The 2010 Panamera will go on sale on Oct. 17. The S model will start at $89,000, the 4S at $93,800 and the Turbo at $132,600.