A 4 door model of the 2 Series has been conceived as part of an expansive compact model strategy. And make no mistake BMW are interested in such a model because it adds consistency to the Gran Coupe series and would be a successful profit unit as well as establishing a RWD narrative for this exciting new segment.
The car is viable because of the modular architecture within the platform.
The car won't appear tomorrow. At least we are looking at a possible mid-late 2015 introduction. BMW need to assess the reaction regarding the CLA and A3 Sedan in regards to how they affect the next step ie C-Klasse and A4 especially in regards to upcoming replacements.
Another factor is at best the car will be a four seater and rear passenger leg room will not be class leading. But as some have said the potential for a four dr is high given the lukewarm reaction to the CLA et all but BMW are confident that the Coupe could hold the segment until a Gran Coupe model enters the market as it delivers on every level where a BMW is expected to and confirms to what this segment actually wants but is not being given.
Edit: The following is just a speculative render by a digital artist.