No 4K support in Youtube in note10+
Did you change the resolution of your display to the max? Try it
Head to Settings > Display > Screen resolution and choose WQHD+ 3,040 x 1,440
Make sure your YouTube app and your phone's software is up to date
The resolutions on the Note10 and 10+ screens are 2080x1080 and 3040x1440 respectively. Even if the content you're viewing was uploaded in 4K, you're limited to the max resolution of either device.