I recently shot some footage with a GoPro, and realized after the fact the GoPro 'chapters' the footage around 4 GB, so I ended up with a number of 4 GB files, instead of one larger file. There are various reasons for this, but in the end, I really wanted one long file, so it would be easier to synchronize with footage from another camera and my audio recorder.

So I found this answer on StackOverflow, which had exactly the commands I needed:

ffmpeg -i 1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate1.ts
ffmpeg -i 2.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate2.ts
ffmpeg -i 3.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate3.ts
ffmpeg -i 4.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate4.ts
ffmpeg -i "concat:intermediate1.ts|intermediate2.ts|intermediate3.ts|intermediate4.ts" -c copy -bsf:a aac_adtstoasc output.mp4

This assumes I renamed my files from the GoPro to 1.mp4 and so on, and I'm in the same directory as those files. In the end, you should get a losslessly-joined MP4 file with the contents of all the video files from the sequence.

This is an annoying interim step for long-running footage from a GoPro—and I've noticed other action cams and dash cams seem to do the same thing.

And unfortunately, you can't just use concat: to join the files together in a one-liner, you have to build the intermediate files first (thus you need triple the original footage's size on your disk to do this trick). You can delete the originals and intermediate files once you confirm the output.mp4 file is correct.