BMW's new 4-series convertible has been spied with its top down for the first time. The new model is due for a UK launch in mid-2014 as part of a radical shake-up of the new BMW 3-series line-up.

BMW has opted to name the forthcoming coupé and convertible versions of the new 3-series as the 4-series to give it greater differentiation from the new BMW 3-series saloon. This is a strategy already mirrored for the 5-series, where the coupé and convertible versions of that car are called 6-series, with distinct styling and driving experiences.

Engines for the 4-series will lean heavily on the range in the new 3-series saloon. These will include the new 242bhp turbocharged 2.0-litre petrol unit for the 428i coupé.

The adoption of the 4-series name for the standard coupé also has implications for the M3. The coupé and convertible versions of the M3 will be rebadged M4 and will be even more driver-focused than their four-door M3 saloon sibling.

The 4-series coupé will be the fifth of nine models to be launched in the new F30 3-series line-up. It will retain its retractable metal hard-top roof, despite pleas from BMW’s American importer for a soft-top to differentiate it more from the coupé.

The M4 coupé and convertible are due later in 2014, using the same turbocharged 3.0-litre straight six engine as the new M3 saloon.