Streaming 4k HDR has a lot more to do with codecs used and the hardware deciding capabilities of the clients. So for the server you just need minimal disk speed and average network speeds if your client devices are good. If you do need to transcode you will want an Intel processor that has quick sync. But I would avoid transcoding (waste of electricity) and rely on clients with good hardware decoding. Hevc, av1 etc. My old Nvidia shield device didn't support hevc and had to replace it avoid transcoding my movies.
This is a solid decoder, cec support sucks but most Android TV devices do. Anything with a Amlogic S905X4 or newer should support everything https://a.co/d/as1T5RY
TL;DR; Plex server can run on a toaster oven. High quality streams are more about the streaming device itself