Oh, and in addition to my first comment I also want to add that screen resolution is very important....TO A POINT.

I wouldn't want to go below 300ppi, because then it visibly becomes less dense.
I wouldn't want to go above 380ppi, cause of diminishing returns where it negatively affects the cpu/gpu/ram and battery life.
The sweet spot is between 320 - 370ppi, at the average distance of 10 inches from the eye.

So with that said, 3840 x 2160 (4K or UHD) is overkill** on such small devices.
And 3200 x 1800 is also overkill.
And 2560 x 1440 is overkill.
And 1920 x 1080, believe it or not, is overkill.

As said before, a 5.7inch device is a phone at its limits.
A device with 1080p display like this is going to be 386ppi.... overkill (but not by much).

So what about alternatives?
Well we have 1600 x 900 (aka 900p), and with it a 5.7inch device is 322ppi... sweet spot !

Okay what about a device like the 4.9inch OtherHalf you mentioned above?
Going with 900p yields 375ppi which is a bit of overkill.
Going with 1366 x 768 yields 319ppi which is at the margin.
Both are going to be equally excellent choices.

That 1366 x 768 is a great resolution for devices as large as 4.9 inches to as small as 4.0 inches.

Going smaller than that, you end up leaving phones... and start gauntlets and smart-watches.
At which point, the ppi, or better yet the is very important.
Because there's a fine line between overkill and battery life, and much less "forgiveness" than on phones (or tablets or laptops). Here the 280-390ppi actually matters a lot.

Why does it matter?
Well, firstly the manufacturers can displace that extra resolution into making screens that refresh better, are more responsive, have more accurate colours and lastly are brighter.
A display with those properties but one that is "only" 900p will infact be visibly better than the average 1080p display.

And not only that, it could save precious battery life.
Technology has moved forward that the software is smarter, the processor (and its parts) use less power and accommodate a larger battery.
We are in the days of having moderately used smartphone last 3 days.... but due to the poor decisions by OEMs, we're still stuck at 1 day battery life. A big kudos to the SONY Z3, which is one of the best devices of today, but even that can be improved.

edit: Please Tell Me If You Agree That 900p is Sufficient For Mobile Phones. Otherwise Tell Me What Your Ideal Mobile Phone Screen Size and Resolution Is.

http://www.slate.com/blogs/bad_astro...esolution.html

**unless it is Virtual Reality.