AFAIK the "migration Assistant" does NOT ever erase anything, so the music is most likely still in your old computer.
users>(you)>Music>Itunes>Itunes Music
If you can look at that folder and see how big it is, and you can burn DVDs on your G4, I'd suggest you do that. 1 DVD will hold right around 4GB of stuff, so if that folder is too big you can burn half of it at a time or something. Peace of mind for a couplea bucks.
ALSO, look in your new computer, in
users>(you)j>Music>iTunes>iTunes Music
And see if there's anything in there.
If you see stuff in that "iTunes Music" folder, but it doesn't show up in your actual iTunes application, you just need to go to iTunes and go to
File> Add to Library
And select that iTunes Music folder. It will add every song in there, and it may ask if you want to add duplicates, and just select NO, so you don't have two of the same song if it's already added to your library.
Hope that helps. It's very doubtful you lost anything.