The 1D IV is going to be superior.
Not because of ISO charts or IQ, because when you put a 1.4x on your $11,000 400mm F/2.8 you turn it in to a 560mm F/4 with only slightly less IQ.

Sunshiny bright days it might be a fair comparison. Night time, indoors and low light situation when you are shooting at F/2.8 and need the speed there is no contest.

Fred Miranda has a review of the 1D X, I just read it today. It might give you some info as well. http://www.fredmiranda.com/1dx-review/