4K is Marketing Hype for TVs and Smartphones, Useful Mostly for Tablet, PC Applications

Up to this point, most advances in image quality (high definition TV, for example) have been about “large screen” viewing. Ironically,  4K and higher resolution video standards (though undoubtedly propelled by large and small screen sales) might actually be the first image improvement standard with greater relevance for tablet or PC screens or “lean forward” applications than traditionally has been the case.

source: rtings.com

Whether there are perceivable advantages for smartphones and TVs is questionable.

The reason is that most consumers, sitting at normal TV viewing distances in their living rooms, media rooms or family rooms, will not actually be able to perceive a quality improvement over HDTV.

Image improvement really requires that users be very close to their screens. In fact, for HDTV, viewers have to avoid sitting too close.

In fact, 4K Ultra HD resolution is not worth it if you are sitting more than six feet away and have a 50-inch TV, since the human eye cannot tell the difference between 4K resolution and HDTV.

One additional caveat: screen sizes are quoted on the diagonal. TV viewing distances might, or might not, refer to actual vertical height.

So “ultra HD” only makes sense if you want a really big screen and plan on sitting close to it. Or, perhaps, if your vision vision actually is “perfect.”

source: appliancesonline

At around the distance of 10 inches, people with average vision can discern an image with ~344 PPI, and people with perfect vision can discern an image with higher resolution up to ~573 PPI.

If one assumes that most people cannot detect an image quality difference greater than 344 PPI, 4K is wasted on any screen size smaller than about eight inches.

Screen Size	Optimal Distance 1080p
25"	3.3' (1 m)
30"	4' (1.22 m)
35"	4.6' (1.40 m)
40"	5.3' (1.62 m)
45"	6' (1.83 m)
50"	6.6' (2.01 m)
55"	7.3' (2.23 m)
60"	8' (2.44 m)
65"	8.6' (2.62 m)
70"	9.3' (2.83 m)