I'm also using a g4 running on 10.3.9.
There haven't been any "software updates" for 10.3.9 for some time now. I'm guessing that there aren't going to be any more.
For all intents and purposes 10.3.9 "as is" is "as far as you're going to get" with it.
You didn't say which Mac you have.
You might consider searching for a 10.4 "Tiger" install DVD and upgrade. Then you can get to 10.4.11. Granted, it won't be "the latest and greatest", but 10.4.11 still runs _almost_ everything out there.
Either buy a "universal" install DVD (installs on any Mac), or search ebay for an install disc specific to your Mac (be careful what you buy here). The universal discs cost more, but if you don't take care and try to buy a Mac-specific disc, you could end up with the wrong one.