I downloaded a 4K video and only sound is playing, no video.

AV1 codec & VP9 codec

YouTube encodes streams at resolutions above 1080p using the AV1 codec or VP9, which are not installed by default on Windows.

You can download them from Microsoft Store:

• AV1 - https://apps.microsoft.com/detail/9mvzqvxjbq9v

• VP9 - https://apps.microsoft.com/detail/9n4d0msmp0pt

The other option is to install advanced video players such as VLC or MX Player.