5DIII to 5D IV? Not really an upgrade. I am a canon fanboy but I am not going to be dishonest about this, Who is shelling out $3500 for this camera when you can get better alternatives for less? If resolution is your thing then by all means go for it but you can get the 5DsR for less.
I am not trying to be a dick, Would love to know everyone's reasoning to get the 5D Mark IV. What feature has you hooked?
Given that an 80D shames a 5D3 for low ISO DR (despite its sensor size disadvantage), and that early reports appear to indicate the 5D4 has made a similar step forward in DR, I'd suggest the 5D4 is quite a significant upgrade vs the 5D3 in that regard (indeed, it's one of my main reasons for upgrading).
If I wanted huge DR (well, OK, I do, but I want other things too) I'd invest in a D810 or an A7Rii. If I wanted huge resolution (at some expense in DR) I'd get a 5Ds. If I wanted huge frame rate it'd be a 1Dx2.
Point being - the 5D series has become a jack of all trades; there's always going to be a camera out there that betters it in some area(s); but as an overall single camera for someone that doesn't really want to run multiple bodies, it does most things very well.
I held off for ages when the 5D3 was released (I was considering jumping ship to a D800). In the end, the 5D3 turned out to be a great camera - with limitations and frustrations, but overall still a great bit of kit. The 5D4 looks like more of the same, and improves in a couple of the areas I felt the 5D3 lacked the most.
Having experienced DPAF on the 80D, I'm also looking forward to that addition on this new 5D.