Follow these steps to combine multiple MP4 files using MKVTools.
Download and save MKVTools from here to your desktop.
Double click to extract the zip archive.
Drag the MKVtools app from the extracted folder into your Applications folder. Then double click it to launch the application.
Click the Tools tab at the top of the MKVtools window.
Open a Finder window and navigate to the location of the video files you would like to join. Make sure that the names end in 001, 002, 003, etc. where 001 will be the first video in the join queue.
To rename a video simply select it and press the Enter key to modify the filename.
Highlight all the mp4 video files you would like to join then drag and drop them into the square box within the MKVtools window.
MKVtools will now join your MP4 files. A dropdown drawer will appear to display the status of your join.
Once complete you will notice the combined video file appear in the same folder as your source files.