Power from the N55 motor rises from 315bhp to 404bhp, and while this is not quite so strong as the twin-turbo 431bhp S55 motor in the M4, its 442lb ft of torque is more than commensurately stronger than the 405lb ft offered by the M4.
It’s kept the eight-speed ZF automatic transmission (disappointingly, and unlike the M4, no manual is available) but modified over 20 percent of its components for faster, cleaner shifts.
On the chassis side, everything – springs, roll bars, dampers, bushes, bump stops – has been changed, but the overall strategy was to increase the springing medium by 40 percent while allowing more damping compliance in bumps and less in rebounds.
Alpina's iconic 20-inch wheels, fitted with Michelin Pilot Sports developed specifically for the car, and vast Brembo brakes complete the picture. Note, however, that Alpina still does not provide a limited-slip differential as standard. It needs it less than a D3 but it still needs it and, at £1890, it is eye-wateringly expensive.
As for the B4's standard equipment the coupé gets dual-zone climate control, adaptive suspension, cruise control, xenon headlights, 20in alloy wheels, a Dakota leather upholstery, electrically adjustable and heated front seats, rear parking sensors and BMW's iDrive infotainment system complete with sat nav and DAB radio. Choose the convertible and the list will be similar to the coupé's except for the folding metal roof.
Expect a savage road warrior, however, and you’re going to be very disappointed by the Alpina B4.
Despite its menacing quad-pipe Akrapovic exhaust system, the engine starts quite softly and turbine smooth. The ride seems a little fussy at first until you realise you’re hearing more than you’re feeling.
Despite liquorice proportions, those sidewalls are actually 20 percent softer than a standard Michelin of this size. If you leave the dampers in their default setting you can waft around all day in quiet and comfort. This has always been a key component of the Alpina proposition, and it’s here again, front and centre.
Nail the throttle, though, and you’ll find another side to its character altogether as in one silken wallop, it flings you up the road. The engine note is a smooth as a 1980s 2-litre six cylinder BMW, its thrust nibbling into junior supercar territory.
The gearbox is good enough to make you wonder why BMW insisted on a double-clutch transmission for the M4 and the overall performance package so strong yet civilised it could be a cage fighter with a first in fine art from Cambridge.
These standards are almost, but not quite matched by the chassis. The good news for skid scholars is that with the limited-slip differential in place, the B4 is beautifully balanced, nicer by far on and over the limit than the far more aggressive M4.
Perhaps more relevantly, ditching BMW’s horridly squidgy wheel for a firm-rimmed replacement provides far better feel. The B4’s weakness is in its damping which, for serious work, needs to have some of the bump dialled back in again.
This is a fine handling car and there was always going to be a price to pay for that secondary ride quality but there is just a touch too much body movement when travelling fast on difficult roads, a characteristic likely only to be exacerbated by anyone taking it to the track.
The Alpina B4 Biturbo, then, is mightily tempting. The B4 over-delivers on the promise of both its looks and what we now expect from Alpina.
Its real skill is to provide a wider breadth of ability than any BMW using the same shell without contriving to be a jack-of-all-trades. It is damn near as fast as an M4, easier to drive, quieter and more comfortable – and while undoubtedly less exciting, ultimately no less satisfying at all.
If it is undone it is not by the M4 but its own, in house rival, the diesel-powered D3, which is available in coupé, welcome please the D4.
What little the D3 loses in performance it more than recovers in torque, range and running costs, qualities than might make it less of a BMW but even more of an Alpina.