There is a MUCH simpler and FREEway to do this.
You have the music from your 4g ipod on your mac right?
Therefore plug your iPod into the PC and reformat it using the iPod software so it's a PC iPod.
Method One
"Next plug your PC iPod into your mac and instead of syncing the itunes library make a folder on the ipod called 'Whatever you want' and move the music from your library to this folder. And then plug your ipod into the PC and copy the music from this folder into the PC's iTunes and your done."
Method Two
"Plug the PC iPod into the mac and sync the library again. Then plug the iPod into the PC again and use some free software...(Don't know I'm a mac user, but I know they are out there.) To 'RIP' the music from the iPod and into the PC's iTunes library."
If you want to reformat your ipod as a Mac pod again after all this then simply plug it back into your mac and use the iPod software to reformat the iPod