Of course you must have install GPAC

<code>sudo apt-get install gpac</code>

Then to combine the mp4 files, use\\
<code>MP4Box -cat 1.mp4 -cat 2.mp4 -cat 3.mp4 -cat 4.mp4 -cat 5.mp4 -new newcombinedfile.mp4</code>\\
where the files after <code>-cat</code> are the mp4 files and the <code>-new</code> forces the creation of a new destination file.