How to remux both video and audio from another source into MKV?

[Gerff]

I've just downloaded a movie, but it's in m4v and I prefer my files in mkv. I would like to know if there's a way I can extract the video and audio streams from that file and remux them into a mkv file without losing much of the original quality.

[Baldrick]

Download mkvtoolnix and use the included mkvmergegui. Open the m4v and make a new mkv.

[Gerff]

Can I do that with mp4 files too?

[Baldrick]

Yes.

[Gerff]

Ok, thanks for you help!