Sure, but the 6 Gran Coupe buyer is not a 5 series buyer to me. He/She's older, probably earned more money, and now that kids are gone, wants something more personnal, particular, different and still sexy.
Maybe this 4 series GC is to compete with Audi A5 Sportback. It's just I don't see much difference between 4 series GC and 3 series sedan, except that it may be sexier. Let's hope it won't be too expensive.