i am tinkering around with an ibook g4. what is the most updated software version this machine can handle. Thank you,

I am quickly messing around with an ibook g4 and I am going to quickly offload it to someone who is content with very simplistic operations. However I would like to make sure I have the most capable operating system loaded prior to passing this machine on. I am not well versed with Mac products and don't feel like spending too much time conducting trial and error education. Could someone please inform me of the best and most up to date OS for this ibook G4. Thank you for your time.