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:
With iMovie (Yosemite) I merged 3 videos made on my iPhone 6s, piece of cake.
in Projects (tab) click +
Select Movie, no theme
Import Media (I had my 3 videos on desktop, Cmd-Click selected all 3 at same time)
Import Selected (presto, imported)
Click on one of the videos then Cmd-a for selecting all 3.
Click Hold-Drag all 3 down to timeline.
If you click on Share, Email you will get 5 choices of sizes to save the video, it will then open an email and place the video in the email. You can drag the video off the email to desktop (or not) and delete the email.
There is a checkbox to add the video to Theater, where all your completed videos are shown.