Sorry, but I can't agree with the statement and especially with that last sentences. A transaxle-platform, such as the one of the AMG GT, doesn't work if you want to have "genuine four seater". The transmission at the read end just eliminates the leg-room in the second row. Porsche tried that with the 924, 948 and 944 while the 928 was advertised as the "big 911" in the late 70s as far as i remember. All of them were two seater, though Porsche's intention was a more practical 911.
Moers already highlited that the GT-platform didn't work for the GT4, so they sticked to the MRA. The CLS was developed by the same people that made the W213, while the AMG GT4 is AMG made. It will be far more dynamic than the CLS could ever be and will probably look a million times better.
It think the GT4 will be called "GT53", "GT63", "GT63S" and "GT73".