I'm a first generation Canon 6D owner and user for about 4 years now. I did fair bunch of photos with it, but I started looking for upgrades.
Few weeks ago the opportunity arose when my multi-control button stopped working after being exposed to light snow. Before jumping to buy a $3000+ camera body I decided to rent one for the weekend and weight the pros and cons between upgrading and repairing my camera. So these are my thoughts on the 5D Mark IV coming from a 6D user perspective:
Image quality is better and you can easily see it during processing. You get way more details out of highlights and shadows with the 5D Mark IV. The colors seem a little bit more saturated, but still maintain the "Canon color" look (with the 5D Mark IV I didn't touch the saturation slider as much as I would with a 6D). It also has less noise in the shadows at ISO values smaller than 1600 (maximum I needed during my tests).
Faster shutter speed is useful in bright daylight, especially for portraits (where you want that shallow depth of field achievable with larger apertures). My Canon 6D with a 24-105mm f/4 L lens can't be used at f/4 during bright days. I have to close down the aperture quite a lot (sometimes to f/6 or f/7). With the 5D Mark IV maximum shutter speed of 1/8000 I was able to use a f/2.8 lens during daylight (not the brightest though) without even reaching its shutter speed limit of 1/8000 and that was pretty awesome.
Viewfinder has a 100% coverage compared to 97% on the 6D. This was something I noticed from the beginning and it's a big improvement in my opinion. I found it very useful when shooting in tight spaces where you want to make sure you get right all the details on the edge and there is no way to apply shot-wider-and-crop later approach.
Another cool thing about the viewfinder is the super-imposed level indicator. On Canon 6D you can map it to the Depth of Field Preview button, but it will use the exposure indicator to show level. That means you cannot see if you are shooting level and check the exposure in the same time. Also, 5D Mark IV also features a 2 axis level visible via view finder, while canon 6D only shows one axis.
Ergonomic are worse in my opinion. The 6D has less buttons and feels well thought. The 5D Mark IV has more buttons, but harder to access. Having the preview and delete button on the left side of the screen, together with a bunch of other buttons made everything complicated. On 6D you are able to hit the preview & delete buttons using only one hand. On 5D Mark IV you have to use the both hands to preview and delete photos. Also related to ergonomics, the 30 grams extra shouldn't be something to worry about - I didn't feel the difference.
The joystick on 5D Mark IV is also less precise than the multi-control key with arrows. Every time I used it felt like it was slowing me down. The button on 6D gives you tactile feedback when you press and you know it was pressed. The joystick on 5D Mark IV doesn't give any tactile feedback and forces you to check the screen when you use it. The placement it's also bad when it comes to change focus points while looking through view-finder (on 6D you can easily change the focus points while looking through view finder without having to move the camera away from the eye).
Having more focus points is definitely better, but I'm still not used to use more than one focus point (center one). Having more cross-type focus points didn't make the difference in my usual workflow. I also didn't feel any difference when focusing in low light - an area some said the 6D was sometimes better than 5D Mark III. With my nifty fifty (50mm f/1.8) I didn't experience any focus sluggishness, which made for a better experience compared to the 6D.
CF storage cards seem to be faster than normal SD cards I use in my 6D. Unfortunately I'm not aware of any laptop that has a CF slot, so you might have to carry a card reader with you. But this might not be a big issue if you connect the camera directly to you computer to download images. The transfer speed from the camera is improved and you can re-use and external HDD cable.
Touchscreen, GPS and WiFi are things that I almost never use. Not saying that they are completely useless, but I would not mind a lighter camera that doesn't have these features.
Dual Pixel RAW was something I had disabled. The need of using Canon software before continuing with my normal Lightroom workflow is not something I'm willing to do. Also, I'm being less and less a pixel peeper and I can not see when I would want use that.
I decided to wait a little bit before upgrading. Ordered a replacement button for my current camera and plan to do the repair myself. If I succeed in my repair attempt I would use the money to buy more lenses instead of upgrading the body. In 2 years I might switch to mirrorless if they catch up with ergonomics and ease of use.