5D4 would be a nice video camera if they had put in a good 4k codec, I actually only set to film in HD in that and not 4k.
Canon has a lot to catch up on in mirrorless but Nikon has even more. One of the most important thing is AF in live view and Nikon just does not have that nailed down at the moment, not even in the D850. At least Canon has Dual Pixel AF, it is great at tracking and finding faces, they they lack Eye-Focus AF which portraits photographers love because getting the eye in focus is vital and this makes it super easy.
The camera game now isn't about mega pixels, it seems we have now settled around 24mp as the sweet spot. All the latest bodies can do 14 stops of DR easily and even the not as good 6D is acceptable.
The game has now moved onto Focusing and who has the best, I rather have noisy photo than a out of focus one so focus and IBIS is the next thing. Canon has already put electronic stabilisation in their M5 mirrorless. They need to put a sensor stabiliser in next and need it fast because everyone else now has it.
I would think Canon is about 3 generations behind Sony, their next camera is probably going to be like the A7Rii. By the time the Canon mirrorless comes out Sony would have released the A7S3, having learnt what they know from the A9, A7R3 and A73. I really don't think Canon or Nikon's first mirrorless will match anywhere near Sony's current line up purely because there is just so many areas that they don't have.
Another major concern is Canon seems to have really long life cycles of their bodies, whereas Sony just throw out a new body every 6 months. They can't really release a 5D mirrorless and not update it for 5 years, that isn't going to work if they want to improve the tech and make ground. There will be the A75 out at that point and it will be so far ahead the photography world would be so far shifted they would have a hard time to catch up.