HT1338: OS X: Updating OS X and Mac App Store appsLearn about OS X: Updating OS X and Mac App Store apps
Currently Being ModeratedJul 3, 2013 3:25 PM (in response to Geoff O)
Yes, you can. Buy a retail Mac OS X 10.5 DVD from a source such as Amazon or eBay.
Currently Being ModeratedJul 3, 2013 4:28 PM (in response to Geoff O)
Note that Leopard is as far as your G4 Mac can go (all further versions of OS X require an Intel processor) and that Leopard will set you back around $200.
You might consider that putting that towards a modern Mac is a better investment. A G4 Mac is officially obsolete and cannot be repaired by Apple.