if your server is powerful enough to transcode, it will transcode 4k content to 1080.
but even if your server is good enough, generally you should avoid transcoding 4k.
additionally, smart tvs, while fine for netflix/etc, do not handle well full 4k bluray rips via plex due to lack of support for HD audio.
so this can cause your 4k tv on plex to transcode the 4k to 1080 in certain situations.
there is a good recent discussion @ Best hardware to play 4k via Plex server
and also check out @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ for a long detailed thread about 4k and its pitfalls.
