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The door shutline is actually covered with an additional horizontal panel across the upper section of the door(evident on up-close pictures)
What you actually see is not the door shutline but a clever ruse to disguise the car.
Recent interior pictures show why the car is actually a 4dr as the rear seat is sitting right at the quarter light which is where it will be on a 4dr if it was a 5dr the seat would be further forward to help with the trunk space.

The Gran Coupe has to sell in the US and the US do not take kindly to hatches. With a 3er GT already in the family , it does not make sense to offer two hatches.

The car is effectively a slimmed down CS Concept in outline with a shorter deck but still an elongated 4er Coupe.

Why a 4dr 4er Gran Coupe? why not?

The 6er Gran Coupe is being bought by customers who enjoy the individuality of sporty driving but they need the extra flexibility , these customers like to stand out too and a 7er , 5er and 3er? Well everyone has one of those.