AFAIK 4K YouTube streams use different encoding now and N6 doesn't have HW support for it. So 1080p or play x264 4k locally.
P.S.
Also now you shouldn't need to put screen res into build.prop as YouTube now supports 4k on 1080p screen natively. Again AFAIK my F2 has no such mod and it works fine.
P.S.
Also now you shouldn't need to put screen res into build.prop as YouTube now supports 4k on 1080p screen natively. Again AFAIK my F2 has no such mod and it works fine.