You could install Windows if you wanted, but you would need to use Virtual PC which is an old application that Microsoft created for users of the PPC version of Macs that needed to run Windows applications on their Macs. Unfortunately that application is REALLY slow and useless. If I were you I would try to get my hands on a second hand MacBook or Mac mini if you don't want to drop too much cash in one shot.

the point of buying this G4, is to get familiar with Mac OS
so I think that I am not looking for something to run Windows on PPC laptop.
but thank you anyway, maybe I will need this advice sometime