Focus and Handling
Looking at a spec sheet, the AutoFocus system on the Mark 3 and the Mark 4 are fairly similar. There are some key differences which I found to be quite useful since I tend to shoot at very large apertures (f2.0 on the 85mm f1.2L and 50mm f1.2L). The first difference is that the autofocus points are spread out over a larger area - which makes choosing the correct point easier.
Autofocus speed is pretty similar to the Mark 3, but lets face it - it was already a stellar performer, even in very low light. Autofocus accuracy however, I feel has improved significantly. When shooting at large apertures I'm noticing a much higher keeper rate which reduces the number of reshoots and allows me to shoot subjects such as streetstyle (where time is critical) with much less time/waste. For the little bit of LiveView shooting I did, DualPixel AF works impressively quickly and tracking is smooth.
The ergonomics of the camera haven't really changed much - the Mark 4 is a little bit lighter, but other than that if you're used to the 5D series, you get the same great user experience and updated Canon menus. Thumbs up.
Continuous shooting has increased by 1fps to 7fps - which is a noticeable improvement... although for sports/birding/action shooters who can afford it, the 1D is still light years away. Reviewing images, camera menus and general usage is very snappy and the camera doesn't feel sluggish at any point. Image transfer is also quite fast.
The Mark 4 is noticeably better than the Mark 3 in this field. There is a small improvement in High ISO noise & colours.. but the main gains are when shooting in good lighting. The colours and dynamic range of the new sensor are simply put - fantastic. The size of the image is also ideal since it gives enough cropping room, without the bulky file sizes of say the 5Ds which takes its toll on all the subsequent workflow.
Finally! the secondary slot supports UHS. This makes a big difference in card speed and the buffers are cleared noticeably quicker. The camera also features WiFi which makes it easy to connect to my tablet/phone and transfer images of the camera. For bulk/dumb transfers during shoots i'm still using an Eye-Fi card since this part of my workflow was already set up.
The Mark 4 feels a bit more sturdy than the Mark 3. I can't give an exact reason why :) (and this could just be a placebo/new toy effect) however it feels very well put together
Great for when you're shooting in LiveView in weird positions... but not really a killer feature for my usages. I still ended up using the wheels for my day to day operations
Dual Pixel RAW
This could be chalked to me being an old fart, however, the increase in accuracy out of the box doesn't really make me need this feature to the point where I'm fine with the increased file size. I used this initially a bit, it's cool but it's something I didn't keep using once the novelty wore off.
I really have nothing much to say in this section. If I had to be picky, the battery life (using the same batteries I had for the Mark 3) is a bit poorer than the Mark 3 but we're still talking about battery life that would last me for a few days of shooting (using the battery grip).
The price has increased a bit over the Mark 3 at launch, however prices have already come down and I think the Mark 4 represents really good value for the workhorse it is.
Conclusion.. a.k.a Should I upgrade?
This is an easy one - Yes. The Mark 4 improves all the core areas of photography and these will translate into improved quality, speed and ergonomics for pretty much any photographer. These will be particularly noticeable if you do ambient light shooting or any sort of reportage/weddings. I really recommend this camera and I'm personally very happy that I've made the switch. Initially, I thought the difference was minor, and that I would use the Mark 4 and Mark 3 interchangeably but ever since the Mark 4 has entered my bag, I have almost never picked up the Mark 3 again... I think that speaks for itself.