There are lot of rumored specification floating over the web, a specification surfaced a month ago suggest that the camera will feature 24 MP sensor, now according to latest rumor the camera will carry 28 MP FF Sensor… so there is a possibility that two or more different prototypes of 5D Mk IV exist in the wild, however two things are common in all the rumored specification till now I have read.. 4K recording option and introduction of Dual-Pixel CMOS AF sensor in a Fullframe Body..
Take a look at the updated list of rumored specification
28MP sensor
Dual Pixel CMOS AF III
New and more advanced AF system (compared to EOS 5D3)
4k video
Anti-flickr technology (seen first on the 7D Mark II)
Improved video features
Crop mode (featured on the upcoming EOS 5Ds)
ISO 100-51200 (H1: 102400, H2: 204800)
The camera is expected to arrive at late 2015, get regular via joining the Facebook Page of 5D Mark IV
Also see – New Canon EF 400MM F5.6L IS Lens Coming
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My prototype has new external flash control capabilities in it, but don’t have 4K recording with 60/50 fps.
Can’t say more.
I’am not sure if we see the new 5D Mark IV (or whatever called) this year, because the camera will kill the 5Ds(r) sales.