You can do this very quickly without re-encoding

Open up Terminal

Command-Space Bar type in terminal to launch it or go to your Utilities folders.

Once Terminal is open you can change the directory to what ever folder you files are in. My example folder is called "Join" If you don't change the directory it will default to your User Name folder.

$ cd /Desktop/Join Hit return

My .mp4 movie files are for example movie1.mp4 & movie2.mp4 Please note that they are joined in the order that you place them in. You can drag & drop them all into your terminal window only after you type in the command "cat"

Here is what it would look like.

$ cat movie1.mp4 movie2.mp4 > movie.mp4
Hit Return and presto your mp4 file is now combined/joined as just one file.

Notice the ">" after my movies thats because when need to join it & name our .mp4 file with a new name which i called mines "movie.mp4"