Im hoping they make some significant improvements for the 5D4. I really Dont care what they do. I have been perfectly happy with the 5D3,s. But , im gonna love these improvements that the 1Dx2 offers. Frankly, you cant buy a bad camera right now for professional use. They are light years ahead of what we did just 4 yrs ago.
I Miss the build and the AF of the 1D series. I like my 5D3. Really missed the 1DMKIV when i sold off and went all 5D3 for our needs. At one point we ran 3 5D3 bodies so its not like I hate the 5D3. I dont. Its a great camera. So hoping the 5D4 sees significant changes. I do not see them crippling the 5D4 but I do see this technology thats in the D5 and 1Dx2 to remain in the top line bodies. D750 is a great camera. I have shot with it. ISO is awesome on the camera. Its Nikons answer to the 5D3 with some improvements over the 5D3.
I think Canon will get the 5D4 another step above the D750for features and AF and i think all the Mfgs will now makes changes to these bodies that their core buyers want and continue to lock them into their brands. I know everybody wants a 5D4 thats going to do what a 1Dx2 does for $2995.00.....I dont think its gonna happen. I think canon will up the ISO another 3/4 stop and get 12800 fairly usable on this next update and probably very similar the the current 1Dx1 as far as sensor performance. But its not going to be the 1Dx2 performance both in terms of ISO and AF features. Its not now with the current bodies and that will continue with this next update.
My guess/realistic hope is that the 5D4 will have what is essentially the 1Dx II AF system - albeit with some deficiencies; probably the same hardware but with some f/8 AF points disabled, fewer DIGIC processors (like the 1Dx vs 5D3), and therefore probably not quite as quick (at AF).
Maybe it'll have 4K video - it would seem sensible given the 5D's historical use in this area. Maybe just 30 fps (as 60 fps MJPEG would be too much for CF cards). Maybe they'll go for CFast and SD, but I suspect CF and SD would be more likely.
Dual Pixel looks very likely. High ISO will be a bit worse than the 1Dx II, and obviously frame rate and buffer depth will be nowhere near.
As for MP, probably 24MP-28MP (I'm hoping the high end). Here's a totally crazy suggestion: the 5D3's 22MP sensor had a resolution (width) of 5760 pixels; exactly three times the 1920 pixels of HD video (I assume this might have been to make downsampling the frames easier). A 25MP sensor would have a width of roughly 6144 pixels - exactly 1.5x the 4096 pixel resolution of 4K video. I understand the 1Dx II takes a crop from the sensor for 4K, but that might be a way of getting a decent full frame 4K image. Purely speculation on my part.