just posted to canon rumors
"We’re told the following specifications are for a camera that may turn out to be the EOS 5D Mark IV, but for the moment it is just a test camera that may never get to market in this form. We’re expecting a replacement to the EOS 5D Mark III some time in the final quarter of 2015.
A camera currently being tested specifications
18mp Full Frame CMOS
61 AF Points (all crosstype)
4K Video Capture
It was stressed that the above is NOT the EOS-1D X Mark II. There will be a quantum leap in fps, dynamic range and a bump in resolution for that camera. We’re told that there’s a possibility that a replacement to the EOS 5D Mark III could be called the EOS 5D X, but no final decision has been made on that.
I stress again that this is a reported test camera that may never come to market, and it is unknown how old this test camera is."
That's no longer a general-purpose camera, but an action camera that can't replace the 5D3. It just doesn't have enough resolution. Although they may have more leeway in that direction - now that they have the 5Ds to take care of those with higher resolution requirements, they can reposition the 5D4 to cater more towards those who don't care about resolution at all.
I'm not impressed by the arbitrarily high maximum ISO number, though. Anyone can position that as high or as low as they like. At high ISO, the DR-to-ISO curve is linear - ISO 6400 pushed four stops will look exactly the same as ISO 102400 on the same camera, and there's no penalty to simply underexposing and pushing if the camera doesn't natively allow for such a high ISO. The only thing that matters is the signal-to-noise ratio, which determines how clean the image will look at any given ISO in the first place.
Also, why push the DR in the 1Dx II, which will be an action camera? It's the 5Ds landscape/studio/architecture/still life camera which needs DR, not the action-oriented 1Dx.