its M4A to MP3
MP4 is a video file (even though M4A is called MPEG-4, M4A is the 'audio' version)
You can in fact do it in iTunes if you change the import settings in Edit > Preferences to MP3, then right click the songs you want to convert, choose Create MP3 version
Would recommend you set the quality in the import settings to at least 128kbps (personally I go for 192 or 256 depending on how many songs/how much space you have available)
To seperate them out, right click the columns at the top and choose File Type, and then youll have a new column that shows if its an MP3 or not
Does that help