As expected, the updated EOS-1D Mk IV firmware v1.0.6 has only one change listed in the release notes. It is said to improve performance of the camera's autofocus tracking function, both for receding subjects and those that are approaching the camera at low speed.
The firmware update file, along with instructions for its application, can be found on the Canon USA website.
The firmware update file is a 12.36MB download, and should be applied only when using the EOS-1D Mark IV's AC adapter kit or a fully-charged battery pack. The current firmware version on a 1D Mark IV can be determined by switching the power on, entering the menu system and scrolling to the end of page three of the yellow Setup menu.