Canon has confirmed that the 1D C 4K DSLR is a 1D X with a firmware update. The only thing hardware wise that has been changed deals with the flash sync jack.
The Canon representative confirming speculation said that a firmware update would not be available for purchase for 1 DX users so they to would be able to shoot in 4K. Tech wise, this is possible, but Canon would be losing money, something no business wants to do. I’m sure there are several people out there working on a firmware hack or a workaround even as we speak (Magic Lantern anyone?). We’ll keep you posted.