Samsung and LG have no short-term plans for 4K UHD smartphones
According to rumors from South Korea, both Samsung and LG will be distancing themselves from 4K smartphones at least for the near future. This leaves Sony as the first (and so far only) major manufacturer to confirm a smartphone with native 4K UHD (3840 x 2160) resolution via the upcoming Xperia Z5 Premium. Thus, Samsung and LG will continue to offer smartphones with displays "only" up to QHD (2560 x 1440) resolution.
The report claims that the two South Korean companies would rather improve upon their existing QHD screens than to continue investing in higher resolutions. Properties such as lower power consumption, improved colors, brightness, and more comfortable readability are apparently taking priority over resolution.
While 4K UHD will inevitably become more common, it currently plays a very minor role in television sets let alone smartphones or tablets. The limited library of native 4K content is also both an issue and barrier for mass 4K adoption. While upcoming technologies like VR can certainly benefit from the higher resolution, they are far away from becoming affordable and ubiquitous.