1Dx has 1-2 stops better ISO ability than a 5D3. I'd be surprised if a 5D4 can improve by >2 stops, enough to put it ahead of a 1Dx. On par, maybe.

My wife and I "share a pool of cameras": two 1Dx, a 5D3, a 5DsR. Her technical habits are too sloppy for her to use the 5DsR in most cases, so she usually gets a 5D3 and 1Dx, and I get 5DsR and 1Dx, though on rare occasions I'll take both 1Dx. Overall, it's been a great setup for us (with a trusty 1D3 as spare, serving time at my folks for an occasional bird photo), though I've wished I had another 5DsR as backup (but couldn't justify it in front of other lighting and lens wishes). 1Dx2 does nothing for me; maybe a 1Dx3 with dual CFast, but the incremental fps bump is of no use _to me_. Bring in the 5D4, and now I start looking at it as the potential double-duty camera: upgrade my wife's 5D3 to 5D4, and becomes a much better spare for a 5DsR than anything else we had. Oops, but wait, the 5D4 price is awful close to a 5DsR, AND I can't reuse my grip...now I'm back to a 5DsR (and steal the grip from my 5D3) unless/until the 5D4 price drops.