if you are collecting 4k, you probably shouldn’t be converting them to anything, the whole point of having the 4k files is the highest quality. Otherwise you should just stick to 1080 bluray and convert to the size/quality that fits your eyes/ears.
that said, if you are converting anything for permanent use (ie rip and store in your media library), then you are often best quality from CPU encoding.
Of course no one can tell you what method/quality works best, only YOUR eyes and ears can tell you that.
the best advice is to experiment, generally starting with the fastest/least quality using small tests, start increasing the quality until you get something that you are happy with.
there are tons of handbrake guides on the internet that can provide you with all kinds of details and suggestions.
for plex streaming/transcoding, GPU is where it is at, plex does the transcoding dynamically/on the fly and the originals are not modified, and the transcoded files are not saved (they are streamed). Since the transcoding on the fly is only for the temporary stream, the quality (cpu/time) is less important than the speed to convert to the applicable bandwidth restriction.
see also @ Plex, 4k, transcoding, and you for a ton more 4k info and discussion
finally, probably the best balance/budget server would be a 7th gen intel cpu + 630 intel integrated gpu. (no nvidia card needed)
this is the minimum needed for 4k transcoding on plex
