How prevalent is the ability (hardware and software) to edit 4K video?
Nearly any modern computer with a decent graphics card can do it, and using free software too (Resolve).
In any case, the relevant question is: who should care about 4K? And my answer is: nearly nobody. Your eyes can't see the difference, at least not at a realistic eye-to-screen distance.
It looks like the A7s will be my next camera, and I'm not getting the external recorder needed for 4K. I'm getting it for the big sensor, the clean image, the DR, and the codec.
I think of it as a really tiny camera that can give me a slightly better image than a C100, with a bigger sensor, better EVF and higher fps, for less than half the price.
(counting on price and DR to be as expected; those could still go wrong)