If you bought it from a friend, then ask them to reformat it for you as a FAT32 partition. Mac can not see NTFS, which most Windows users use since you get more space on the drive.
If it is brand new and your G4 isn't recognizing it, then I would return it for a new one. I don't trust things that aren't working out of the box, especially when they charge so much for things.
Answers & Comments
If you bought it from a friend, then ask them to reformat it for you as a FAT32 partition. Mac can not see NTFS, which most Windows users use since you get more space on the drive.
If it is brand new and your G4 isn't recognizing it, then I would return it for a new one. I don't trust things that aren't working out of the box, especially when they charge so much for things.