Files of 320 kbps or less that have the m4a file extension are equivalent to mp4 or mp4a (mp4 audio file.) It is a newer & improved format compared to mp3. And m4a files with a bit rate of higher than 320 kbps will be apple lossless (similar to flac & monkey audio or ape files.)

More about m4a files can be found here http://en.wikipedia.org/wiki/M4a. Whilst this article suggests there is no difference between mp4 audio & m4a files, WinAmp has created a slight variation of the format meaning it has a variation of the mp4 audio format. Both of winamp's mp4 audio file formats are basically the same. Edit: if you want to know more about apple lossless audio format, see http://en.wikipedia.org/wiki/Apple_Lossless