There is a massive difference between the 5D3 and 5D4/EOS R files. Period.
I have all three and shot a wedding last weekend with all three of them. The 5D4 and EOS R have nearly identical image quality and character. But the 5D3 is not close - as far as I'm concerned.
When I first got the 5D4, I didn't notice a huge difference in the files when compared to my 5D3's. But after using the 5D4 for a couple of months and getting used to processing the files, I began to cringe whenever I needed to use the 5D3.
Is the 5D3 a bad camera? No - It's fantastic! But to say there is not a noticeable improvement in IQ - especially without hands on experience - is 100% disconnected from reality.
The 5D3 has no concept of blacks and shadows. And you are forced to ETTR. I needed to relearn how to shoot when I got the 5D4.
The dynamic range is better from ISO 100 to 25600. Noticeably better. When I dump my files into Lr or C1 for editing, 99% of the time I can tell which camera I used by the SOOC image quality. Think about that.
In the 5D4, the colours are better, truer, and most important - malleable enough not to fall apart during processing. The whole editing experience is harder with the 5D4, because you have such a depth of image data that did not exist in the 5D3 images. You have options and expanded processing latitude.
Again, maybe the 5D4 isn't a necessary upgrade. But it is most certainly an upgrade.
Of course, this is all just my opinion. I shoot portraits, products, weddings, boudoir, and architecture professionally. But not wildlife.
Thank you Sir- From another 5D4 and 1Dx2 and 5D3 owners who shoots 25+ weddings with all 3 cameras every year. I put my 5D3 away when he receptions start. Its no where remotely close to the 5D4 in ISO performance and DR
And sorry Teamspeed.....There is 3 full stops on a 5D4 over a 5D3 especially when you factor highlight recovery and ability to push blacks and shadows with the normal exposure push. EASY 3 STOPS more DR