See here for m4p conversion FAQ:
I have converted the music I downloaded from iTunes from M4P to MP3. It works PERFECTLY !!!! The thing that's good, is that once you converted them, you can put too many MP3 songs in your MP3 player!! What I mean is : I converted a song (A Storm Is Coming from the Lord Of The Rings). The size in M4P was 2860 KB and in MP3 it's only 995 KB and the quality did not change !!!
If you want a proof, just send me a song in M4P and I'll convert it to MP3 and send it bakc to you !!
Let me know and I'll tell you how that works. It's a piece of cake!! Don't bother yourself looking for MP4 converters!!