Extract the subtitle file and convert to mp4 with ffmpeg.

ffmpeg -i file.mkv

Look for lines mentioning “Subtitle” with a stream number like Stream #0:2 or similar.

Once you know which stream is the subtitle you want (for example, 0:2), extract it with:

ffmpeg -i file.mkv -map 0:2 subs.srt

Then convert to mp4:

ffmpeg -i file.mkv -c:v copy -c:a copy movie.mp4
Rename the subtitle file same as mp4: movie.srt