I ended up figuring this out through trial and error, but I figured I'd put the settings that I used here for posterity. In include/config.php the following allowed me to preview mp4s:
$ffmpeg_audio_extensions = array('mp4');
$ffmpeg_supported_extensions = array('mp4');
$ffmpeg_audio_params = "-acodec libmp3lame -ab 64k -ac 1";
I also added:
$ffmpeg_preview = true;
I assume this is necessary, but it wasn't included in config.php by default. In addition to these config options I removed the "-strict -2" setting in $ffmpeg_preview_option. I looked through the ffmpeg documentation and could not find it so I'm not sure if this is vestigial or if I missed it, but my setup seems to work now.
Is there a better place to look for configuration settings than the "knowledgebase?" It has good general information, but doesn't go into depth at the level that I would like.
Thanks,
Joe