Then you cannot use 4K on your TV with the file you have.
As soon as you have both subtitles activated and you choose a different audio track than the first in the file, Plex will transcode the file.
This is a workaround because otherwise video and audio will “drift” apart and synchronicity cannot be maintained.
All you can do is to:
- convert the subtitle into SRT format
- remux the video file, so your desirted audio track is the first audio track.
see also Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ