https://www.sony.com.hk/en/electronics/support/articles/SX906801
Xperia Phones' display resolution
The Xperia phones conform to the Sony 4K definition of 3,840 horizontal pixels (in landscape orientation). With a 21:9 aspect ratio, this results in 3,840 x 1,644 pixel resolution.
Watching 4K video in 21:9 aspect ratio
The Xperia phones let you enjoy 21:9 aspect ratio content, edge to edge and without letterboxing. This is optimal for movies created in 21:9 format and available in 4K resolution.
Playing back recorded 4K video in 21:9 aspect ratio
You can record 21:9 4K video using Cinema Pro, and play it back in true 4K quality on your Xperia phones.
4K is activated in 4K movie playback on YouTube, Cinema Pro etc. 4K playback depends on apps specification.
Watching non-4K content
Xperia phones use advanced upscaling technology to display non-4K content, enhancing contrast, colour and clarity.