...or, it could mean that there's no operating system installed on the hard drive at all.
That PowerBook G4 originally came with "Software Restore" CDs or DVDs, which contain the requisite operating system that the PowerBook originally shipped with. These CDs or DVDs can only be used with that particular PowerBook, and, therefore, should follow the PowerBook from owner to owner.
I recommend kindly asking your brother to give you those CDs or DVDs, as they're useless to anyone other than the current owner of the PowerBook.
There's a good chance that his reply will be, "I lost them." I'm still trying to figure out how people completely lose track of the original software CDs/DVDs that come with their computers: those CDs/DVDs are to your computer as keys are to your car, and people manage to keep track of those just fine.
Sometimes Apple is a great help in supplying you with new "Software Restore" CDs or DVDs -- other times, like when a computer is several years old, they're not.