I think owners of the 5DIII that would consider "upgrading" to an imaginary 5D4 are probably people with really deep pockets and lots of disposable income- so more power to you! As a professional artist I try to keep gear expenses to necessity. I can't see how dropping another 3k+ (If I owned a 5D3) would be a wise move.
And since Canon has released 3 relatively new FF bodies in recent years (1DX, 5D3 and 6D), I really can't see how it would make much sense for them to release any new bodies with incremental upgrades.
Who would they be targeting with this new camera? I can only see a need for a high mega pixel camera in Canon's lineup.
Wouldn't you agree, people with deep pockets, and a lot of disposable income, are usually a very good demographic to market to???...I'm just saying! Besides, if photographers, like yourself, are happy with their current gear, you're not going to upgrade regardless of the specs of a 5D MkIV anyway.
Canon needs to sell camera bodies every year, so they're probably going to market to those who want the "latest and greatest" technology, and are willing to pay for it. I'm one of those consumers.