https://trash-guides.info/Radarr/Radarr-Quality-Settings-File-Size/
I just use the trash guides I don't do 4k and I have it as low as DVD because a lot of cartoons don't get picked up when I had dvd quality disabled.
To change the quality just make multiple profiles, i have one for 4k and one for regular. Default is regular but sometimes rarely I'll set a movie to 4k and then just hit the search button and it will upgrade it or downgrade if I went the other way