I write this review from the perspective of an advanced photographer who does the occasional video. For three and a half years, I owned the 5D Mark III; its responsiveness and the image quality it delivered with a good lens made it less a camera and more an extension of my mind and eye, faithfully recording the grand landscapes, events, and decisive moments I was privileged to see. The upgrade to the IV was more a personal choice, rather than from being compelled by any major limitation of the III. I’ve used it now for more than a month, and feel I know enough to write a review.
A 5DIII user will find the controls very familiar; I was able for the most part to start using it without referring to the manual. This is a good thing - all that muscle memory you built will remain useful. The shutter feels softer and quieter, a design carried over from the 5DS/R to reduce vibration. The viewfinder phase detect AF is everything you expect from a camera like this - I had no problems with my 24-70 II, 70-200 II, 135 or 85 1.8 at maximum apertures. Tracking appears improved, although I need a bit more time to say definitively. Perhaps the biggest surprise is the touchscreen. After a month, I find myself using it more and more in preference to the joystick when navigating the menu. Dual Pixel Autofocus is as as fast as advertised - certainly as fast as viewfinder AF. What I found pleasantly surprising was that I seem to be able to get sharp shots at exposure times well over 1/focal length seconds using this method. You do have to get past the ergonomic considerations of shooting with a heavy camera held at arms length though, and it is probably better used with smaller lenses versus something like a 70-200.