Bottom line, for wide angle 4K video with the MK4:
-Tokina 11-20mm f/2.8
18-30mm FF equivalent with wide aperture, great built. Great optics. Best option right now, but 599$
-Sigma 18-35mm f/1.8
29-58mm FF It's wide on my 7D APS-C which is identical to 5D crop. So if you don't need ultra wide angle, and want the best optics, this is a GREAT companion for the 5D4K. This specific lens is VERY popular among video shooters, even quite really popular on high-end cinema/movie sets, 799$
Not much competetion beyond these two lenses is present currently (at least far as we video people know, of course unless you need cheaper, then it's the Canon 10-18mm EF-S)
(I don't know you call ''wide'' but anything below 35mm on crop I see being wide personally, 24 is wide, 18 is really wide, 11 is special-purposely-wiiiiide)
So go Tokina or Sigma or get a Canon FF wide if you'll use it for FF Stills in a serious manner. 16-35mm EF lenses are the most obvious tempting example. Bear in mind that Slowmotion is FF, and maybe you'll want a shot with shallower DOF so FF, or take 30mp stills FF. That's why the 16-35mm lens type is tempting.