Maybe this car is shorter. It is a 2 door for one and it would look better shorter. They also want to seperate it from the 3 sedan. Being physically different also helps to establish another reason for the increased cost as it is usually more expensive from a production numbers view to have two different wheel bases.
Also it kind of makes sense. 4 series 2 door shorter on its own wheel base, 3 sedan on its longer wheelbase, and 4 Grand Coupe on the stretched 3 series LI wheel base. It starts to visually really separate the cars.
I like it as with the light weight suspension the M version is supposed to be getting and less body to haul around the M4 could have a nice weight loss. Also the 4 series would be a more nibble handler with the shorter wheel base.