Converting MPEG-4 (m4a) to WMA or MP3?

My girlfriend has bought a new CD player for her car and it will play music from a USB stick but the music has to be in MP3 or WMA format. Most of our music on the laptop is in MPEG-4 format because that is the format that i-Tunes converts songs into. Does anyone know how to convert it back to WMA or MP3?