This command will not work with a mp4 with embedded subtitles & extracting subtitles from an Mp4 will result in MOV_TEXT style subtitle file, which is not what you want. Basically, google for an SRT, test it out and then we are ready to go.
If you have embedded SRT you also need a seperate file with just video and audio. To strip the srt streams, leaving the just video and audio you want.
ffmpeg -i movie.mp4 -map 0:0 -vcodec copy -map 0:1 -acodec copy movienosubs.mp4
So lets assume you now have
The FFMPEG command would be this: (Note this only copies the first audio stream, you may need to mess around with mapping streams if you wan't to get more complicated, but you get the idea)
ffmpeg -i movienosubs.mp4 -i subtitle.srt -c:v copy -c:a copy -c:s copy movieFINAL.mkv