Beneath its slinkier, sleek body, the D4 has the same twin-turbo six-cylinder diesel engine as the D3, the same chassis and suspension design, the same eight-speed automatic gearbox and even the same steering and braking systems.
The only areas in which it differs from the D3 Biturbo, in fact, are these: it has two fewer doors, a slightly lower-slung driving position, a retractable metal roof and a price that’s four-and-a-bit grand higher.
As for standard equipment both the D4 Biturbo coupé and convertible get dual-zone climate control, adaptive suspension, cruise control, a quad-pipe Akrapovic exhaust system, rear parking sensors, lashings of burled Elm wood, automatic lights and wipers, electrically adjustable and heated front seats, a Dakota leather upholstery, xenon headlights and BMW's flawless iDrive infotainment system. Our test car rode on Alpina’s optional 20-inch wheels, which look great, and was adorned with optional stripes, which don’t.
Beyond these subtle differences, though, we are talking about the same car fundamentally as the D3. Which means that you get a 3.0-litre straight six with 345bhp and 516lb ft of torque.
It’s mated to ZF’s ubiquitous but superb eight-speed automatic gearbox. And that’s enough to fire the 1585kg D4 coupé to 60mph in 4.6sec, to 100mph in about 10 seconds and to a top speed of 173mph, while the convertible amazingly manages the same trick despite its weight deficit. This is a diesel car, remember, not some hunk of fire-breathing mid-engined Italian exotica.
The D4’s best party trick, however, is its ability to deliver such thundering levels of performance while returning more than 40mpg in the real world, and a touring range of almost 500 miles.
Fair enough, if you thrash the living daylights out of it, the D4 might return as low as 35-36mpg (shock, horror). But if you were to drive a factory BMW M4 in a similar fashion, it would be at least 10mpg if not 15mpg thirstier.
In theory, then, the D4 should drive identically to the sharp-handling, smooth-riding, monstrously rapid D3 Biturbo, and in most respects it does. But there are slight differences in the detail, some but not all of which are welcome.