Will the higher ULTRA game resolution be wasted on a non 4k TV?
No, I use ultra on a lower res screen, and it looks better than the 1080 setting. Just switch between them to see the difference. I think it is more the quality of the textures that are set with that option.
Same here, it does make the graphics better, even if logically it makes no sense
" It will use nicer lighting, nicer shadows, and additional polygons to give things more detail."
source: https://support.zwift.com/hc/en-us/community/posts/208792423-Ultra-Graphics-Settings