A few days ago Canon announced a firmware update for the Canon EOS-1D C, which, among other fixes, brings 25fps at 4k. The firmware is not available by download, instead you have to take your EOS-1D C to an authorized Canon service center in order to get it on your camera. This is unusual.
The Phoblographer reports about users that are suggesting Canon doesn’t want the firmware to get in the hands of EOS-1D X owners. Why? Because the hardware of both cams is nearly (if not totally) the same and the EOS-1D C firmware could turn an EOS-1D X in an EOS-1D C, a higher end gear with a much heftier price tag ($11,999 vs the $6700 of the EOS-1D X). Btw, the EOS-1D X also got a firmware update in the last days.
The Phoblographer is currently investigating the issue with Canon.