MourningStar:
Download this single file...
http://ffmpeg.zeranoe.com/builds/win32/ ... -static.7zExtract ffmpeg.exe from the bin folder. That's all you'll need.
'Mux' your two files together with the following command...
ffmpeg -i video.mp4 -i audio.m4a -vcodec copy -acodec copy final.mp4
This will very quickly join the two files without any 'processing', thus maintaining the original quality.
There's almost nothing that can't be done with ffmpeg. It's the engine that drives almost all video editors out there.
Note: you can compress the exe with UPX to reduce the filesize to about 10MB.