I posted this a while ago.
1. I get all 61 AF points with my 100-400 II and 1.4 III. I love to use all the AF configs and Case numbers when I shoot and I not yet convinced Cases 5 and 6 do much for me unless I am using one of the zone AF modes. I'll be using that combo a lot more now.
2. The focus is very snappy even with the TC. When I was testing on the birds on the shots I was taking I was impressed.
3. A few case number parameters have more steps for fine tuning. Identical to the 1DX II.
4. The 5D4 has a separate AF area selection button just beside the multi-controller button. The 7D2 had it as part of the multi-controller button. If Canon had offered it as a retro for my 5D3 I would have paid to have it installed. I paid to have Canon install a locking mode dial retro on my 7D.
5. I did not really consider it but the touch screen is pretty amazing. I can whip through the menus. I barely use the green my menu's now. When I pick up my 7D2 I find myself tapping the screen.
6. When you press the Q button you get the Q screen. Press info and a second Q screen comes up that shows all the AF points, ISO, etc and you can program more things you want to see when you open that screen.
7. I have been shooting at 32,000 ISO and getting good results.
8. The 5D4 offers the Detail Set similar to the 7D2 when you are programming the AF-ON and * buttons. You can get a few more things like a specific case number, AF mode, switch to High Speed Continuous to auto engage.
So all around it has everything my 7D2 has that I wish my 5D3 had, more megapixels, better high ISO, better AF than both 5D3 and 7D2 and it is FF.