Test-Movies/
/Ant-Man and The Wasp (2018)
/Ant-Man and The Wasp (2018) [1080p].mkv
/Ant-Man and The Wasp (2018) [2160p].mkv
/Await Further Instructions (2018)
/Await Further Instructions (2018).mkv
If you have to move them anyway, get down in that file system and examine each file’s properties. I use MediaInfo (freeware). Right click the file, select Media Info and presto, all the info regarding resolution, audio format, subtitles, etc.
Why are you trying to do this IN Plex? Just go to windows (or whatever OS you are using) and do a find on all the movies with that 4k marker in the name and cut and paste them into a new folder. Create a new library in PLEX for just that folder and reprocess.