I transcoded a DVD into several
m4v files with HandBrake. How can I merge these files into one
You can use the built-in iMovie app to do this.
Create a new project, add your video files, and drag them to the timeline in order. Export your project as an M4V (Share-> Export Movie).
If you're completely new to iMovie, some good introductory tutorials can be found on YouTube.
Join can use MP4Box.
On the shell: