BMW has confirmed that the next-generation coupé version of the 3-series will become a stand-alone model and be badged 4-series when UK sales start late next year. It has just taken a bow in public for the first time at the Detroit motor show.

The significant change to BMW’s line-up is previewed by this Concept 4-series Coupé, which finally lifts the lid on one of the car industry’s worst-kept secrets: that the two-door 3-series will be become the 4-series and join the 6-series as a two-door variant of a BMW saloon.

“The ‘4’ doesn’t just mark the start of a new cycle; it marks the start of a model with its own individual character and a stand-alone design,” says BMW.

Although billed as a design study, the 4-series concept is understood to be a faithful representation of the production car. Only some of the more extreme elements, such as the flamboyant front and rear bumpers and air dams, will be toned down.

These first official pictures show the new two-door to be a muscular and broad-shouldered design draped over a wide-stance platform.

BMW has released the key dimensions for the concept, which are expected to be carried over to the production car with very little change. They include a 2810mm wheelbase that it shares with the 3-series saloon.

At 4641mm long, 1826mm wide and 1326mm high, the new concept is 17mm longer than the saloon, 15mm wider and a significant 67mm lower, a dimension that emphasises the two-door’s sportier footprint.

The front and rear tracks reflect the squat look, too. At 1545mm, the front is 3mm wider than the saloon and the 1593mm rear is 10mm wider.

Despite the brief for a sportier, more individual look, designers have crafted bodywork that broadly follows the theme of the 3-series saloon, with a distinctive swage line running along the bodyside and linking the front and rear haunches by running through the door handles.

Where the 4-series concept asserts its identity is in the execution of the swage line, which takes a sharper, more aggressive form and mirrors the more pronounced sill cover. The wheel arches of the 4-series concept bulge more aggressively than those of the four-door. These body adornments also lend BMW’s smaller coupé a more aggressive look than its bigger brother, the 6-series.

At the front, the family resemblance to the saloon is carried through, with headlights that link into the hallmark kidney grille. The grille itself is made more prominent by protruding more aggressively from the surrounding sheet metal.

“The front end displays a clear link with the 3-series family, but with a sporting take on the theme with precise contours and multi-faceted surfaces,” says BMW.

The execution of the intersection of the grille and headlamps is sportier thanks to the narrower lamp units built around on-trend ‘squircle’ projector lamps.

Underpinning the front is a gaping front air dam that extends the full width of the nose. The production coupé is tipped to have a more toned-down airdam, although elements of this design may influence the forthcoming M4.

Another departure for BMW is the hockey-stick-shaped decoration on the front wings, behind the wheel arches. Some sort of adornment will appear on the production 4-series, but how close its design will be to this feature is unclear.

The interior is based closely on the 3-series saloon’s and, ignoring the concept car decoration in this design study, appears to be directly carried over.

The instrument pack, minor switchgear and instrument panel’s upper moulding, for example, mimic the 3-series saloon, with its high-mounted infotainment screen. Likewise, the radio and air-con modules in the centre console are carried over from the saloon.

BMW hasn’t released any chassis or engine information with the concept. But our sources say that the production 4-series will feature engines selected from the saloon range and mirror the bulk of the line-up in today’s 3-series coupé.

Today’s best-seller is the 320d, which will morph into the 420d. Seven further engines are likely to result in a 418i, 420i, 428i and 435i petrol line-up and 425d, 430d, 435d diesels. The long-awaited M4 is also a key 4-series variant.

Chassis settings are tipped to be firmer than the saloon’s to further establish the 4-series as a separate model.