I don't think it ever made sense to say it's generally hard to tell 4K from 1080p. It just really depends on the viewing distance and size of the screen.
My tiny laptop screen is almost 4K, and the difference is massive from a 1080p screen because I'm so close.
Likewise, your 55" inch screen wouldn't need to be 4K if you sit 10 meters away from it.
Well, as I think I may have said poorly, I don't really know if I can tell 4K from 1080p reliably, because a 4K stream that contains such high quality pixels in it that it qualitatively different than an upscaled 1080p stream is very hard to come by right now.
According to the charts, I'm right on the outer edge of where it may make a difference, so it's at least physically possible I could tell, if I got a truly 4K stream.
The upscaling process on some televisions is astounding. Upscaling a 1080p@24 source to 4K@120 display brings out so much detail and gives the content an entirely different feel.
http://www.rtings.com/tv/reviews/vizio/p-series-2016 has a 120hz panel but at 4K the "120hz" is interpolated; it's not actually getting 120 updates per second from the source (I wish). I have this exact TV as a monitor and 1080p gaming @120hz is glorious. 60hz is fine for desktop use but now that I've tasted it I can't go back.
It's not just linear interpolation, there's all sorts of clever edge detection going on as well. It still cannot invent detail out of thin air of course, but based on the type of contents it can infer the detail that "could have been there". YMMV though, I hate it just as much as motion compensation.
I went from a 2001 Toshiba 720p flat CRT to 2016 55 inch 4K SUHD HDR. I guess my brain had to get over the shock.