It was speculated based on screenshots and whatnot, but for those wondering if it is real, unfortunately it is. Sony has since confirmed it with a statement to PhoneArena in which they state, “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.” This more or less confirms earlier speculation that the switching between 4K and Full HD was done to conserve battery where the handset’s battery life is promised at around 2 days.
Also given how there are reports that overheating is not an issue with the Sony Xperia Z5 lineup, we speculated that this was a possible reason, along with the use of dual heat pipes to better help dissipate the heat generated by the Snapdragon 810 chipset.