YouTube will play 4K videos in 1080p

The problem is that the hardware of your TV is not strong enough to process VP9, the proprietary video coding format developed by YouTube-owner Google specifically to handle Ultra HD video. On the contrary, 4K videos on Amazon are compressed via HEVC, not VP9.

To play YouTube videos in 4K or higher resolutions, a YouTube 4K converter may help you convert the formats or resolutions into something your TV can accept.