To get the most from a 4K display, you also need a very large room; VK member ChrisW6ATV must have a living room the size of the state of Texas if his flat screen is over ninety inches.
I live in a one-bedroom apartment and could not hope to fit a 92-inch (!) 4K television in here. My own flat panel is 19 inches, and I sit ten feet away from the set. The TV is connected to the cable system by means of coax, and I am not using a cable box; this means I am definitely not seeing anything in HD on my TV, even though the set itself is capable of 720p resolution. Even my Blu-ray player, which is connected to the set via an HDMI cable, may not be showing true HD either.
As far as movies are concerned, I have several films on standard DVD in my collection, but I haven't watched any of them yet. Most of what I watch on DVD (and cable) are classic television series from the '60s and '70s; it is extremely unlikely that these DVDs were produced in HD, as the programs themselves were not HD to begin with, but old, low-definition (!) 525-line NTSC.
I don't think, either, that the classic-TV networks on cable (MeTV, Antenna TV, RTV, et al.) are showing very old classic television series in HD, since the programs were made under the old NTSC standard; upscaling these old shows to 1080p HD will not result in true high definition, although the effect, barely noticeable though it may be, will be there. I get three classic-TV networks on cable, but none of them ever advertise that their programming is presented in HD--because it probably isn't, as I am about to explain. It is almost certain that the classic shows these networks put out are nowhere near HD when they finally go out over the cable, regardless of how much the format is fiddled with behind the scenes.
VK member walterbeers has a point as far as the sharpness issue is concerned. If it is in fact true that a 4K display can produce a sharper picture than the human eye can handle, I too would wonder who would need such a thing.
I think, since every time we turn around the industry is coming out with larger and larger LCD displays, and the sets themselves can do more than any ordinary television (even the first flat screens) could possibly do in the pre-HDTV era, owning a TV these days is (or could be, in some very competitive neighborhoods) a lot like "keeping up with the Joneses"--i. e. if someone has a 50-inch flat screen and his neighbor, for example, gets a 60-inch or larger set, the first person will get a 65-inch-plus TV, just to keep up. The neighbor will then get a 70-inch TV, prompting the other person to get an even larger display; and on and on it goes. The only way it (the competition) would stop would probably, even likely, be if one or the other gets a TV so big it crowds the family out of the house.
I can actually see something like this someday happening, as flat screens are getting larger and larger all the time. It won't be long, IMHO, before we see 100-inch and larger flat screens available for home use, but I'll bet those monsters won't fit in average-sized living rooms; as well, the price tags will be so high that very few people could afford them. Not to mention that a 100-inch flat panel TV weighs something on the order of (I'm guessing) about 200 pounds or so, not unlike the large 3-way entertainment-center console CRT TV-stereo-radio combos of the '60s through about the '80s (think Magnavox with their 25" "stereo theatre" combination set of the mid-'60s; that set must have weighed the proverbial ton).