How can I merge .m4v files?
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:
MP4Box $(for file in single*m4v; do echo -cat $file; done) joined.m4v
Related: https://superuser.com/a/364159/95878