I don't own a 1D Mk IV (yet), as it is really out of my budget considering the use it would get (occasional sports photography), so I got a used 1D Mk III instead.
However, a pro photographer I know, always has 2 1D bodies with him on shoots, one main body and one backup, and the way he works he uses one for a year, which then becomes the backup body, and the old backup body gets sold on. He buys a new 1D body simultaneously, whichever is current at the time, and this replaces the old main body. IOW, a two year switch and replacement plan. He has been doing this for many years now, but this year he broke from this pattern.
He got his first 1D Mk IV, and a little over 3 months later replaced his remaining 1D mk III with a 1D Mk IV as well, basically because he was dumb-struck by the IQ increase. Noise levels are definitely a lot lower, especially at higher isos, and for his work he just loves the increase in IQ ay higher isos. He also thinks AF is a lot better, but for him 1D III AF was good enough already.
If you read Clarkvision, you will find that the 1D Mk IV is actually a trend breaking camera with regard to the sensor. It has the lowest noise of them all, wel outside the established pixel size progession envelope.
Kind regards, Wim