What a load of crap. I've never heard of m4a files having anything of the kind. I think you're confusing m4a with wma (What you should know about WMA Files)
Quote from the link I gave earlier which discusses m4a files:
"Apple Computer started using and popularizing the M4A file extension to denote the file was an unprotected (non digital rights management) MPEG 4 Audio file."
If m4a files can have viruses then so can mp3's. Find me proof to your statement than offering an opinion based on imagination.
m4a is in fact a superior compression technology format to mp3. If you didn't know that then you don't know much about what you were talking about.