Essentially just copy the existing video and audio stream as is into a new container, no funny business!
The easiest way to "convert" MKV to MP4, is to copy the existing video and audio streams and place them into a new container. This avoids any encoding task and hence no quality will be lost, it is also a fairly quick process and requires very little CPU power. The main factor is disk read/write speed.
With ffmpeg this can be achieved with -c copy. Older examples may use -vcodec copy -acodec copy which does the same thing.
These examples assume ffmpeg is in your PATH. If not just substitute with the full path to your ffmpeg binary.
