M4A is another name (actually, the file extension) for the AAC audio format. It is used in iTunes, but frequently has compatibility problems when used outside of Apple-land.
Does your car player expect MP3 format? If so you can convert your files by following this guide: http://support.apple.com/kb/HT1550
Also, unless you enjoy doing format conversions, you also have the option to buy your music in MP3 format in the first place, from another music download store, e.g. Amazon MP3.
MP3 can be used in iTunes, iPod, iPhone and just about every other player, program, and device.