Reading and Writing m4a iTunes files

The FAQ says that mp3tag supports mp4 filetypes. Are files with a .m4a extension (such as those created by iTunes) mp4 filetypes, and if so should mp3tag be able to read and write tags to them? I've got some m4a files but mp3tag doesn't show any tags. I wondered if they were corrupt, but J River Media Center does show tags (but of course, can't write/update them.) Thanks for any info.

Yes, m4a is a mp4 file type. iTunes uses the m4a extension for audio only mp4 files. Mp3tag writes these type of tags and is iTunes compliant. I cannot speak for J River, though. Not every player reads all iTunes compliant tags, so if you don't use iTunes then check the FAQ or Help file of your favorite player.