I was not referring to 4K content exclusively, but any content encoded in H.265/x265. I was simply commenting that Direct Play of HEVC is not limited to LAN.
But since you are experiencing issues with 4K, you should check out the rules of 4K (specifically rules 1, 2, 8):
If you really need to downscale a 4K video, you should probably look into curating your media (for example using handbrake) to create different file versions instead of making Plex do it on the fly.
To your point of streaming on 4G or Wifi, that’s why I compress an HEVC SD (480x720) version of my Blu Rays. Direct Plays like a charm, and does not require much extra effort.