4K isn't as stupid as some make out. Most films are shot or converted from film at 4K so it should be fairly quick to get a new BluRay format based on h.265 using 4K which would give a plentiful supply of films in the format. Broadcast TV on the other hand isn't going to shift for probably a decade. Likewise there isn't much point in 4K sets under 50" but for high end if the studios/manufacturers don't expect the mass market to move to the new format no one will end up disappointed.
The point isn't a technical one, but rather that human biology is simply not able to take advantage of such high specs.
It's the same argument I make against a retina Thunderbolt Display any time soon. It's almost exactly half the ppi of a 13" rMBP, at the same native resolution, and just over twice the screen size. The rMBP, "retina" being an undefined marketing term referring to some threshold where pixels become undistinguishable at the usage distance, is used more than twice as close to your face as you would a Thunderbolt display.
Hence, in practice, a Thunderbolt display, at 2560x1440 is
already retina. Using arbitrary but probably representative #s, you might use a rMBP 13 about a foot away from your face (226.98 PPI), but since you sit about least two feet away from the screen of a TB display (111.81 PPI), the effective PPI would be equivalent to 223.62. This is the general idea behind how an iPad can be considered "retina" when it is substantially lower PPI than an iPhone, as an iPhone in theory will be used closer to one's face.
Now let's extrapolate this basic mathematics analysis to the living room. Screens are much larger and the resolutions are generally lower (720p and 1080p), but the viewing distances are many, many times farther away. Ultimately, unless you are very, very, very close to the screen (or your screen is preposterously large, and I mean for most living rooms, 80"+), you will not be able to distinguish between 1080p and 1440p or 4K.
This is the limitation of the human retina. Your eyes are only so good. I can cram a 12960p 16:9 screen into an iPhone (if it existed), but it'll still only look as good as the current one because my eyes just can't resolve the pixels anyway. And neither can yours, unless you're sporting a MIT Media Lab bionic eye.