Hey Ghengis,
That's kind of a complicated question to answer, unfortunately. A lot of things to consider, like I was saying in the previous post. You should first figure out what kind of software you want to run on the G4, along with your budget.
If you're looking to run DAW software with multi track audio and MIDI sequencing, there's so many choices out there... Pro Tools (what I use), Logic, Cubase, etc... You should find their websites to look up the system requirements for their programs, and see how your machine stands up. Like I said before, the main thing to consider for compatibility is processor type and speed. I'm sure that most of the current programs are going to require at the very least a 1Ghz G4. Especially if you're gonna be running it on OS X, which is pretty much all them , by now. Pro Tools seems to be the most "power hungry" of them all, but they have lots of cool plugins, both effects and virtual instruments. But all the major DAW's out there can do great stuff as well.
You didn't mention which kind of G4 you have, Power Mac, iMac, iBook, etc. The Power Mac G4's are super-upgradeable, if you want to go that route. I think Power Mac G4's can recognize up to 1.5 GB RAM. RAM is important for running stuff like software samplers and such... If you have one of the old Power Mac G4's slower than 1Ghz, you're definitely going to want to upgrade the processor, which is fairly simple to do yourself if you're handy with a screwdriver, haha. The good news is that there's a lot of G4 processor upgrades available that can pump it up to about 2Ghz, with single or dual processors. iMacs or Mac laptops' processors aren't upgradeable. The bad news is that the upgrades can be a bit pricey for the higher speeds, especially with the dual processors, so you'll have to compare the costs of upgrading to buying say a used G5 for example, which is what I did last summer. Or, even a new MacBook or iMac with the Intel Core Duo processors, which are really fast.
So to sum up, depending on what your needs are to run your software of choice, you can either upgrade your G4, or sell it and get a newer, faster machine. That said, if you're going for the "latest and greatest", I'd say get something newer. Anyway, once you figure out which is most cost efficient (and hassle-free), your choice should be pretty clear. Good luck with it...