The Kia Stinger is in that segment and looks great. As ugly as it is, the new Accord has a fastback look to it too but its FWD.
Look at the rear closer. The AMG GT and the Panamera have hatches instead of the usual luggage lid that makes them a 4 door coupe as opposed to a sedan. The Stinger and Accord still have luggage lids and so does the Mustang, so in order for a 4 door version of it to actually fit that segment I believe they might have to revise it to a hatch. I'm really not sure. It's a weird segment that I dont think has a lot of set rules, but more what the manufacturer wants to claim so their car doesn't seem redundant.
But on the other hand, the Mustang has always been a fastback/coupe though so the luggage lid on it is a coupe design element so adding two doors should still allow them to claim that segment.
Im wondering if Ford is exploring this route, in taking after the Europeans, in order to help bolster sliding sales in Europe as well?