How three into four will go, leaving one No. 1 in the open-top charts

Arguably the open-top car most aspired to on the market, BMW has launched the new 4 Series Convertible (from £36,680) - formerly 3 Series Convertible.

BMW has tidied an un-Germanic anomaly in its range by giving all sporting two-door models an even number: odd numbers cover saloons, Touring estates and Gran Turismo hatchbacks.

The 3 Series Coupe and Convertible had been out of step with their everyday sisters for some time anyway; no longer simply two-door spin-offs, they had morphed into a lower-slung design with few saloon body panels carried over.

And, elegant though the current 3 Series is, the new 4 Series stands even further apart from its erstwhile sister and is even more the dream car for the thrusting young executive and entrepreneurial set.

The big talking point is the fully electric folding hardtop. Its the second generation 3/4 Series to get one and BMW has taken this performance art a stage further.

It raises or lowers in just 20 seconds, tucking away neatly into the boot after the lid has risen before closing again.

Admittedly, in roof-down position it robs boot space, but BMW has now made it easier to deposit luggage beneath the folded sections. And ‘roof up’ liberates the space again, meaning you could go off for a break or holiday with all your clobber, dump the gear at the hotel and get that roof down.

With roof up, you would also hardly know this was a convertible; refinement is excellent, making this a truly usable all-year-round proposition, with the reassurance of knowing it is more secure and less prone to vandalism than a fabric-roof soft top.

But top down is what it’s really about and it’s a good place to be, even at high speed. Keep the windows up and the wind deflector in place and buffeting in front is minimal, although being in the back of a four-seat convertible on anything but a scorching day is not for the faint-hearted (nor for anyone vain about their hair-do).

But I’m a convertible fan, so it’s all windows down, bring on the turbulence and moderate it a little if you wish by directing heat up from the footwells.

The BMW is also a rewarding drive, to be expected when its fixed-roof sister is one of the best sporting coupes around.

With some extra strengthening to compensate for the lack of an attached roof, rather than a folding one, the open-top ‘4’ is perhaps less nimble on the corners and there is a not-unexpected loss of rigidity when the roof is down, but this only shows when the car is subjected to patched road surfaces.

Engine-wise for now there are two petrol and one diesel offered. The test car loaned by Broad Oak BMW of Canterbury and Ashford featured the 184bhp 420d diesel; economical (combined fuel consumption at least 53mpg) yet powerful enough for a 0-62mph time of 8.2 seconds.

There’s also a 245bhp four-pot petrol 428i (from £37,020), but BMW aficionados will no doubt set their sights on the 306bhp 435i petrol (from £44,980) featuring the marque’s renowned straight-six format. n