A 4K screen on a phone is not a trivial change involves having to have a much more powerful hardware to keep moving with guarantees that amount of pixels, and also affects the autonomy of the team, but it is an inevitable trend, also driven by the virtual reality.
I explain apart from the virtual reality helmets that cost a pretty penny, there are many companies that want our phones are the eyes of those helmets, approaching the experience more affordable. Samsung and Google are the most interested companies it either with solutions cardboard, plastic well. But the reality is that they are functional, with phones that are already on the market.