Sony was one of the first companies to get fully invested in 4K technology. They integrated it in their high-end TVs and some other products were compatible with the increased resolution. In the mobile market, the Xperia Z2 was the first device to be able to capture videos in 4K resolution and all of the successors of this phone can do that too. Their newest approach was to include a 5.5-inch display with 4K resolution in their recently announced Xperia Z5 Premium. Most people would argue that a display with a density of 806 ppi is an overkill with benefits that are very hard to notice and that it would only consume more power than any other display.
Some companies have included 2K displays in their recent flagship offerings, but none of them is as dense as the one from the Z5 Premium. The Galaxy S6 includes a display that at 5.1 inches is considered a tad small, but with the 2K resolution it results in a density of 577 ppi and we all know this particular phone doesn't excel in battery life. Apparently Sony manages to still offer 2 days of battery life under normal usage, the company even makes fun of others that can't offer this. Granted, the 3,430 mAh battery of the Z5 Premium is larger than the one included in the Galaxy S6, but the 4K display has 4 times more pixels.
We had reported that to achieve this, the Xperia Z5 doesn't show 4K content at all time and now, Phone Arena could get an official statement from the company confirming this information. The statement reads "Xperia Z5 Premium features a 4K display with a resolution of 3840×2160 pixels based on SID Standard and enables all video and image content to be enjoyed in 4K resolution. All other content is displayed at 1080P or lower resolution in order to optimise the performance and battery stamina for this device, ensuring you can enjoy the 4K resolution when you need it most."
It's pretty self-explanatory, but it means that the Xperia UI that runs in top of Android and other elements are still displayed in Full HD just like in some other phones from the company, including the flagship Xperia Z5. So users will see the display in all of its glory in some apps like the Gallery or with some native 4K content, such as the videos captured by the camera.