Here is the command I am using to combine multiple videos:
ffmpeg -i 75_540_38HQ2.mp4 -i 76_70_20.mp4 -i 76_173_80.mp4 -i 81_186_35.mp4 -vcodec copy -acodec copy M
Assuming you want to concatenate the movie, you can use the following command:
ffmpeg -f concat -i inputs.txt -vcodec copy -acodec copy Mux1.mp4
With the following text in inputs.txt:
file 75_540_38HQ2.mp4
file 76_70_20.mp4
file 76_173_80.mp4
file 81_186_35.mp4
Note: some distributions (like Ubuntu) do not have ffmpeg in their repository and instead define ffmpeg to be an alias of avconv. This won't work with avconv, so in such a case you have to compile ffmpeg yourself. You can check whether you have the real ffmpeg by running ffmpeg and checking if the first output line ends with "the FFmpeg developers".