BMW has never built a high-performance M version of the 4 Series Gran Coupe. The Bavarian automaker has struggled to make a business case for the car as it would cannibalize sales of the M3.

There were rumors that the current-generation 4 Series will spawn an M4 Gran Coupe to compete with the Audi RS5 Sportback, but BMW still hasn't officially announced plans to build one. To fill the void, Alpina is working on a new B4 Gran Coupe, which will be the next best thing to an official BMW M4 Gran Coupe.

For the first time, our spies have spotted a prototype Alpina B4 Gran Coupe getting a workout at the Nurburgring, though it's difficult to tell it apart from the regular 4 Series Gran Coupe as the design changes are subtle.

Camoflague is covering up the rear, where Alpina'ss signature quad exhaust system indicates this is no ordinary 4 Series Gran Coupe. The prototype also has larger brakes and firmer suspension. As development continues, the B4 Gran Coupe will be upgraded with subtle front and rear spoilers as well as Alpina badges.

Our spies were unable to snap any photos of the cabin, but the interior will be enhanced with more luxurious materials and illuminated door sills. Under the hood, the B4 Gran Coupe will likely share the same twin-turbo straight-six as the Alpina B3, which produces 462 horsepower and 516 lb-ft of torque. This will be a significant boost over the regular 4 Series Gran Coupe, which produces 320 hp and 330 lb-ft from a 3.0-liter turbo inline-six in the range-topping 440i.

The 3 Series-based Alpina B3 isn't sold in the US, however, so we'll have to wait and see if the B4 Gran Coupe will be available in the US when it debuts next year.