Because Plex is a bitch and often doesnt play nice when you have different resolutions of the same movie/episode. So in order to keep things working properly (as in, only transcode when absolutely needed, or never) a lot of people have their 4K content in a separate library from the other stuff. And then they only share the 4K libraries with those users who are capable of directplaying it for example. This avoids a lot of issues. And as a result of that you also have separate 4K instances of Radarr/Sonarr so things dont get mixed up.
Of course someone will comment “but it works perfect for me duh!”
Because Plex is a bitch and often doesnt play nice when you have different resolutions of the same movie/episode. So in order to keep things working properly (as in, only transcode when absolutely needed, or never) a lot of people have their 4K content in a separate library from the other stuff. And then they only share the 4K libraries with those users who are capable of directplaying it for example. This avoids a lot of issues. And as a result of that you also have separate 4K instances of Radarr/Sonarr so things dont get mixed up.
Of course someone will comment “but it works perfect for me duh!”