Something to keep in mind when making your decision is that, unless you’re going to invest in a very powerful CPU, hardware accelerated transcoding is likely going to be required (for clients incapable of directly playing your 4K content). And that is a Plex Pass feature:
https://support.plex.tv/articles/115002178853-using-hardware-accelerated-streaming/
Another option would be to maintain two copies of your media: A 4K original for playing on 4K-capable clients and a separate 1080p version for other devices. You can use Handbrake (or a similar application) to pre-transcode the 4K version to 1080p so that the server doesn’t need to do it in real time. Your 4K content could live in a separate library or you can store the two copies in the same one and select the appropriate version at playback time. Obviously, this option requires additional storage.
Finally, this thread has some ideas about how to work with 4K content:
