M4A files are files that Apple uses for music encoded with AAC audio codec and stored in the MP4 container. You can safely rename them to MP4.

iTunes Plus offers music files that have higher bit-rate and that are DRM free. Such tunes cost the same 99 cents, so there is no sense to get copy-protected files for the same price and remove DRM protection from them.

If you can't find some tune at iTunes Plus or you have some audio files downloaded some time ago, you can use lossless and lossy DRM removal software.

There are only two question:
- Do you have Mac OS or Windows?
- Are your M4A files DRM protected ('coz DRM protected songs at iTunes have M4P extension) ?