Jonathan Gomes

Youtube's app for tv has settings menu. There you can find a "clear cookies" option.

When I first got my 4k TV, the videos would always play on the highest available resolution on the youtube app. After some time the app started to present low resolutions. To fix that: Clear your cookies on the app's settings. It solved here, playing again the videos on the highest available resolution . If you're logged, you will have to log in again after that.