Microsoft's implementation here is PERFECT!!! I have 3 small kids and learned long ago that the only way to get photographs where everyone looks okay (or at least where no one looks horrible) is to pull them off of video. I can't tell you how many times I handed a camcorder to a stranger and asked them to just point it at us, assuring the assuring the good Samaritan that it was already on and there were no buttons for her to push. I even used to have a set workflow of shooting an hour of video and pulling the good photos off that video. THAT is the intended implementation of 4K video here. You're pulling 8 megapixel stills instead of 2 megapixel stills.
This is NOT intended to be used for regular video recording. I saw an interview with one of the Microsoft camera engineers this morning where he explained that the S800 processors could handle only a few minutes of 4K video anyway before they would overheat (precisely how long depending on ambient temperature, humidity, etc.). So no, this is not meant to shoot an hour-long recital - the hardware couldn't even handle it, and even if it could, storing and mass viewing 4K video isn't really practical in 2014. This is about pulling stills and unless I happen to catch something truly extraordinary worth archiving for a lifetime, this is how I'm going to use the feature - I'll shoot 4K only in that "please would you mind aiming the camera at us?" scenario and then delete the 4K video after pulling the desired stills.