I recommend 10.5 as it will enable the G4 to function at it's most advanced level. You can purchase the retail OS X disks from dealers (Expensive) or from eBay (reasonable) On eBay search for Mac OS X 10.5 Leopard in software.
Two kinds of disks will be offered. The gray colored ones are those shipped with a specific Mac computer and are not compatible with anything else. The retail disk which will install on any G4 Mac are dark purple with a large black X. They come with all the drivers needed and can be updated to the last and latest (which is 10.5.8) via software update or manual download.
FYI there are two paths to change Mac OS X software. One is an update which is free to download and makes a version the latest available (for instance update from 10.5.1 to 10.5.8).
The other is upgrading to a later newer version (for instance upgrade from OS X 10.4 to 10.5) Until 10.9 Mavericks a person had to purchase an upgrade.
http://en.wikipedia.org/wiki/OS_X
http://www.ebay.com/sch/i.html?_odkw=mac+os+x&_osacat=0&_from=R40&_trksid=p20455 73.m570.l1311.R3.TR4.TRC1.A0.Xmac+os+x+10.5&_nkw=mac+osx+10.5+leopard&_sacat=0