I am not going to say that 4K is TOTALLY useless(because it is), but people need to know one thing: Actual resolution of screen in not big problem for battery, most of power is taken by backlight, and backlight is still the typical size of 5.5". Problem start with rendering stuff on the screen. Because of crappy Material Design that its made from useless amount of layers and almost invisible shadows Its taking loots of resources to make it smooth, And 4k is huge amount of pixels that need to be processed by CPU and GPU all the time, even when you just scroll menu.
Its nice to know that its possible, but think about it like a car:
Trick is not to put v16 engine in small stylish car. Trick is to make it run in every day situation and not make it suck 50L of petrol per 100km and every time when you speed up its run out of gas after 10 minutes.
You need to spend some time and prepare every other component of it to run it smooth and good. And for now we have stupid rush to put more and more in actually old as heII devices that barely run system by it self.
Because that how our devices run: they need to loots of power to run system by it self. I know that windows is different platform but when I can run windows10 on old netbook with old Atom CPU and 1GB of RAM and its run smooth even when I run older game like Alien vs Predator 2 that to be more funny, still look better than most of Android games. I can make fun of it and its a bit sad in the same time.
The problem with 4K on a smartphone-size screen is that it’s either overkill for normal use, or barely adequate for VR, which is already ridiculously demanding for something as underpowered as a phone SoC even at 2K. There’s a reason Oculus wants people to be packing near-top-of-the-line PCs for the consumer Rift.
So here I am, wishing manufacturers would continue pushing pixel density so Oculus VR and the rest have better display options to choose from for future HMD revisions and making things like Cardboard and Gear VR even better going forward, and yet knowing full well that 1080p at those sizes is good enough for any use outside of VR because even my eyes have a hard time telling the difference at normal viewing distances.