Canon has announced that it is preparing a firmware update for the EOS 1D Mark IV.
Quoting Canon:
"As a result of further enhancement, Canon is working to release a firmware update for the EOS-1D Mark IV. This firmware update further enhances the AF tracking performance and is scheduled to be available for download from the Web starting from late January.

We will update this announcement on once this firmware update is ready for download.

Details of the enhancement:
Firmware Version 1.0.6 enhances the AF tracking performance of the EOS-1D Mark IV for receding subjects and for subjects that are approaching at a low speed".

Link to the source article:
http://www.canon-europe.com/Support/...=tcm:13-710446