Here's how to check if your built in chromecast supports 4k

by albus_severus_nyc

Enable stats for nerds on your youtube app on your phone from which you are going to cast to the tv. Then cast any 4k video to the chromecast. Select the 3dots and select stats for nerds again. The tv will now show the current and optimal resolution of the video.

If you have a uhd Netflix sub, you can use the video testing patterns to achieve the same.

tangochili

Curious if Netflix app on Android TV shows the nerd stats. If so how

Speedbird844

The "Select the 3dots and select stats for nerds again" thing is actually in the video page of the video you're playing, and you must tap the thumbnail (it should be greyed out with the text "playing on ChromecastXYZ") for the 3 dots to appear.