This article is basically what I have found so far comparing 1080P/I material on a 4K TV.
http://www.cnet.com/news/can-4k-tvs-make-1080p-look-better/
On the 4K tv's I could afford it might be a step backwards, I could tell our local channels on cable did not look as good on a 4K tv than on a nearby 1080P capable TV. On a very expensive Samsung and Sony they looked at least as good, but way out of my price range, and keep in mind even a cheap 1080P TV looked about as good as the very expensive 4K
when showing cable channels. There was a middle of the road (cost) 4K LG TV that did look good with cable oddly perhaps better than the expensive sets I mentioned. I will caution, getting the settings correct for any TV can make a huge difference and the TV's in a store like Bestbuy are not set up correctly though in the Magnolia section some stores do make an effort to get the picture settings better. Some TV's are better out of the box than others and on the showroom floor therefore look better. But get the settings calibrated correctly and some others may end up looking better. Those that say the 4K showing satellite/cable can look better than a 1080P TV may not be imagining it, as once they get the settings done in their home the picture may be very good. How well a TV upscales plays a big role.
If I was buying today I'm not saying I would not buy a 4K but I would have to be convinced the picture for non 4K material looked at least as good as a less expensive 1080P TV.