Well, if you're talkin'about the SidCard, probably the only difference compared to the c64's is a slight frequence table shifting (btw, it leaves tunes inalterate in facts...)...
Hence, if you download the SidWinder music editor from ftp://c64.rulez.org/pub/plus4/Music
the packer code that follows the main program asks optionally to pack the music either for +4 or c64.