when the 1DXII came out it seemed all the preproduction versions of the camera had an embargo on posting RAW, think there was talk that canon might have even made them straight out of camera JPG only. Probably the same thing here ... won't get real samples until the camera ships.
it is extremely processor intensive to capture video on a 30MP sensor and then down size it to 4K and move it on to the memory card. Plus i believe there is greater potential for moire.
if video is captured from only the center portion of the sensor, the MP count is much lower and the whole process is more efficient.
not sure but I think sony does the same thing with the a6300
Given the amount of general processing involved in modern cameras I must say I'm a bit surprised it's not possible to down sample the whole frame. That said, the 5D3 was exactly 3x 1080p (with a little cropping) so I wonder if that was done to make processing easier.