I have had a Plex server up and running for many years now and have seen the video quality from different homes that I share my library with, and can say that I do not see any or much difference between 1080 and 4k. I have my server set to direct stream and then will transcode if need be. My library is pretty large and no one has complained about quality or speed, and why should they since I allow them on my library.
The last thing I would touch on is that I do not use plex for my personal use if I am at home. On my local network I use Kodi which I feel does a better job of gathering info for titles and tv shows. If I really need to watch something out of my network I can cast from phone to a TV but that would be extreme use for me.
Everyone has their own ideas on how to use their media and I am satisfied with my choices.
Good luck on your adventure.