ffmpeg -i original.mkv -acodec aac -b:a 400k -vcodec copy new.mp4
Replace original.mkv with source file name and new.mp4 with your new files name.
This will just pass through the MPG4 video for to the mp4 container without any compression and convert the AC3 audio to AAC at 400KB/s. (AC3 is not suported with mp4 container).
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