You'll need to get a older version of the Mac OS, such as Leopard.
A power book G4 is a PPC machine (older generation of motorola CPU), nothing above Leopard will work on it.
All Apple OS's from Snow Leopard and up require a machine with an Intel CPU.
Apple ended support for these older OS's that run on PPC machines, your best bet to find one is Ebay. Make sure you get the retail copy, otherwise you may not be able to install it.
There is are two versions of the OS - the restore disc and retail disc.
You'll need to retail version because it will work on all Mac's, the restore disc won't as they are specific to each model of Mac.