It depends on what you want. If you plan on keeping two different libraries in Plex I would also keep two Radarr instances, but if you are going for a combined library you could stick to one and just use a different quality profile for the movies you want to be in 4K.

I do that since I like to have both libraries separate in plex and have duplicates.

1080p for remote streaming and low end devices, 4K for local streaming and approved remote clients only. that minimises the frequency of a forced transcode 4k-1080 stream, when a better looking 1080p native file should have been available.

a second radarr instance lets me manage that separately, but I have them synced to eachother.

Yeah I run 2 instances of sonarr and radarr. It makes updating a bit fucky but works pretty good