Like any other 2016's flagship, LG G5 can capture UHD quality videos with 3840× 2160 resolutions. To record and take 4K videos on LG G5 using the main camera, do as follows:
1. Open the Camera app on your LG G5
2. Make sure the rear camera is enabled and then hit the Gear icon on the top left to open the camera's settings.
3. Swipe to end of available options, and tap the Camera Quality button (the icon should be either 16:9, 4:3 or 1:1)
4. Now look at the second raw and select the UHD button from available options to record a video