m4v - MPEG4 video
I have not seen mv4 files embedded inside other files, but File Juicer should be able to extract them should it occur. It will be identified as a mp4 extension though.
iTunes is set as the default application for m4v files, to sync them with the iPod video.
You can create m4v files with iSquint which is faster than Quicktime Pro. iSquint is a special edition of ffmpegX compiled specially for this purpose.