95% of what this server will be doing is media management and Plex, I’m the only person that uses Plex in the household so will only ever be one stream at a time. Where possible watch 4K, usually large file size remuxes, it’s not uncommon to be streaming 40-75Gb files.
If that’s an E3-1230 v3 (4c/8t 3.3 GHz), you probably won’t be able to transcode 4K material with it. From what I can find this CPU has a passmark score of approx. 9600 (cpu benchmark). To transcode a 4K SDR HEVC video you’ll need a score of approx. 12000 (17000 for an 4K HDR HEVC transcode).