Sony's upcoming flagship, the Xperia Z5 Premium boasts a 4K display, but new information from Sony has confirmed that not everything will be displayed at that resolution. Instead of displaying everything at 4K, Sony has made the decision to only render media, such as pictures, video etc in 4K, and the rest in 1080p. The decision was made for several reasons. In a statement to PhoneArena, Sony said:

Xperia Z5 Premium features a 4K display with a resolution of 3840x2160 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.

The decision makes sense, as it likely isn't worth the processor strain, and battery drain to display things like the Xperia UI in 4K. Instead, Sony has opted to put the resolution to work where it would matter most for users, in the camera and galley, and during video playback. The Z5 Premium is listed as a "two-day battery," and this is one of the ways in which Sony is able to make that a reality.

Source: PhoneArena