- for 4k/hevc transcoding you need an intel 7000 series or newer with 600 series integrated gpu, or nvidia 1050 or newer
- no it is currently not possible to disable transcoding just for 4k. the best solution for you is to keep your 4k content separated in different movie library and do not share the 4k library with non-4k users.
see also @ Plex, 4k, transcoding, and you
