As previous rumors, Canon will release a new firmware update for Canon EOS 5D Mark IV in April, 2017. Now, according to CR, a new feature C-Log is probably coming to EOS 5D Mark IV with the new firmware update.
According to CR, a couple of people that there has been active communication within Canon about C-Log being added to the EOS 5D Mark IV via firmware, and that an announcement could come at NAB 2017 next month in Las Vegas. Along with C-Log, there will several bugs fixed and other features in the firmware update. Stay tuned for more info.