A friend of mine has a 60" 4K TV, LG brand but I don't know the model.
When it arrived we mounted it on the wall and tested it with a few "movies" and tested it as a second monitor attached to his desktop.
Finding movies in both 1080p and 2160p was not easy but a lot of porn is available in many resolutions ... so out came the credit card and we download two porn "movies", each in 2160p, 1080p and 720p.
Sitting on the couch about 2.5m from the TV we couldn't tell the difference between 1080p and 2160p. However, when we paused the movies, the still images were much better from the 2160p source files than from the 1080p files.
We could easily tell the difference between 720p and 2160p content but that appears to be largely a result of the way the TV scaled up the video. Further evidence for was that the 720p content looked better when we used software to scale it up to 2160p and then played the new 2160p clip. Ie., we had two 2160p clips: one downloaded at 2160p and one scaled up from 720p by the editing software: the scaled up 2160p clip was not as good as the native 2160p clip but it was much better than playing the 720p clip and letting the TV do the scaling.
When watching TV provided by the cable company there was no benefit from the 2160p TV - all of the HD content provided over cable was a mix of 1080p and 720p.
When using the TV as a second monitor for things other than watching movies/porn the benefits of 2160p resolution was the most noticeable: text and images were crisp and clear. My friend had previously has a 60" 1080p TV as his second monitor, with a 30" 2160p monitor as the primary monitor and there was a huge benefit to having the resolution of the new TV match the resolution of the primary monitor.
I don't think that porn, regardless of resolution would be shot on a caliber of camera that would really help the difference in resolution matter. Sure there are tons of video recorders that capture 4K and above but that doesn't mean its a 4K recording of similar quality to a better film alternative or proper cinema level rig.
What I guess I should really say is I've easily seen a difference on screens that shouldn't show a difference if using proper source images. To be most direct (give the dirth of quality 4K video sources) the easiest way to compare image quality to me is with a video game. Something like a DOTA 2, that was my experience. Seeing that isometric game in 4K was AMAZING. Like a picture. It was on a 4K 27" panel... who new that you could even sense that leap, I sure didn't expect 4K to matter in anything under 30 inches. Diablo 3 sure doesn't scale well cause of the character assets being such low quality to start, but the worlds look amazing. Star Craft 2 is a great strategy game to see in 4K as they are continually improving the assets in that game. If you like isometric PoV games then 4K is a huge boost.
Shooters on the other had suffer from greater draw distance restrictions that are ever more apparent as resolution increases. So you end up waiting for new rendering standards in those games for the most part before they matter. 3rd person games don't fair much better.
But given currently that isometric games of different sorts are my favorite I hope we see a better distribution of and support for 4K.