In both final cut and imovie you can work with source files. There is no reason and no benefit to convert. Even when you work in a so called 444 space in final cut there is no benefit at all once you finalise in h264 unless your footage was 444 or 422 to start with. This is based on the fact that you can't add in information that is not there in the first place. If you however do convert your footage to an intermediate format (should never do that unless your workstation is too slow) there is a mild benefit in case you want to do extensive grading
I have read a lot of material in and around the idea that downscaling 4:2:0 4K results in 4:2:2 1080p but it's just theoretical stuff as scaling also means information loss.
I don't convert the source files. I can see with my own eyes FCPX 422-444 produces noticeably better results than iMovie 420 whether the export is in ProRez or h264. The better codec allows much better grading and effects - the image quality simply hold up better.
In fact I don't know of any professional NLE that does not process in 422 or higher internally.