in https://github.com/matplotlib/matplotlib/blob/master/matplotlibrc.template#L478, the default codec in the matplotlibrc file is set to "mp4". This is not a codec, it is a container format. If you run "ffmpeg -codecs" and "ffmpeg -formats", you will see mp4 listed as a format, not a codec. Perhaps this was supposed to be "mpeg4"?
cc: @dopplershift