there are several different types of subtitles, not all devices are designed to use all of them (entirely up to the manufacturer/designer).
some subtitles are text, some are image based (pictures).
finally, any time that plex must transcode the audio, and any subtitles are enabled, plex will transcode the video, it does this to keep the audio/video/subtitles all in sync.
for 4k content, it takes much processing power to transcode, so normally you want to avoid transcoding 4k as much as possible (ie direct play).
if you are trying to play a 4k bluray rip, then 2 biggest problems for smart tvs and devices are HD audio (truehd/atmos) and SUBTITLES.
see above for why transcoding audio + subtitles = bad news for your server.
for a lot of 4k info, please see @ Plex, 4k, transcoding, and you
