I finally had a chance to look at the details this weekend and I have to admit I'm not all that impressed; especially given how much time has lapsed since the 5D3 was released. There are a few nice additions like built-in GPS and WiFi, slightly faster continuous shooting, etc.. It's unclear so far in the previews I've seen what (if any) improvements there are in dynamic range and whether they've fixed the shadow noise issues the 5D3 had. It's also kind of surprising they they stuck with CF as one of the storage mediums. I understand the backward compatibility thing for upgraders but that storage medium is really starting to fade in the marketplace. Unless they are going to change their pace of upgrades, CF will be all but dead before this model gets refreshed.
I gave up on Canon 2 years ago (after 15 years as a Canon shooter) and sold off my 7D and 5D3 and all the lenses and switched over to Nikon. I was fed up with the poor dynamic range, shadow noise, low resolution sensor and the fact they they were just not moving the ball forward at all. Many thought I was a fool and should just wait for the 5D4. Now that 2 more years have passed since then and we finally have the 5D4, I have absolutely no regrets on making that decision for my needs.
If you're a portrait or maybe a wedding shooter I'm guessing this is a nice evolutionary upgrade but I don't find it terribly impressive for my needs. I'll be really curious to see how the real world users (not previewers) feel about when they get there hands on it in a few weeks.