Canon’s firmware update for the

EOS 1D Mark IV is available for download from Canon immediately.

What’s the update do you ask?

This firmware update (Version 1.0.6) incorporates the
following enhancement:
Firmware Version 1.0.6 enhances the AF tracking performance.*
*AF performance has been enhances[sic] for receding subjects and for subjects that are approaching at a low speed.

Right on, Canon.