I guess Canon got fed up with people bypassing the firmware on their cameras and allowing uncompressed HDMI output on the 5DIII for instance.
Canonrumors asked about a third party firmware for the 1D series. Here's the reply they got from Canon:
Third Party Firmware?
I was told by someone at Canon that they would “bring the might of its legal team” to anyone that attempts to modify at the software level, the features of an EOS-1 camera body. So I think the firmware community out there today will probably leave the EOS-1D X alone.
More to come on this topic I’m sure. I am hopeful that someone provides pictures of the inside of the EOS-1D C in direct comparison to the EOS-1D X.
From that statement I get the impression you could very well shoot 4k video with the 1D X is someone were to modify the firmware. Of course the 1D C is twice as expensive and Canon really wants people to pay the premium. Of course they also talk about the heatsink inside the 1D C but anyway.