I've rented the 5D4 four times since it's been out. (I own a 1Dx and a 5Ds.) To be honest, I hated the 5D3. I'm not sure why, but I just didn't feel it was that great; probably spoiled by my 1Dx in the handling and features department. My 5Ds is an evolution of the 5D3, but much is forgiven since it has such tremendous resolution and, especially at 100 & 200, very smooth skin tones.
All that said, I really like the 5D4. Though again, it's not a huge upgrade, more a gradual evolution, I like the handling better, the AF is better and the files are better (in my mind and use). Each of the times I've rented the body, I've used it on week-long location and studio shoots. I've seriously considered buying it and bringing it on certain shoots rather than my 1Dx, but I'm in the middle of upgrading some of my older lenses, so it will have to wait.
Essentially, if you liked the 5D3, I think you'd love the 5D4. Is it a "must have" upgrade? That is answered with the question, "Is 30MP more useful to you that 22MP?" For me, that answer is yes. I seldom do video, and in fact, wish Canon would produce a camera sans-video so I'm not paying for something I rarely use. If video is big factor, someone other than I can answer that better.