First of all, which G4 are we talking about here? The all-in-one iLamp, or the big tower?
I have personal favorite uses for both.
If it's the PowerMac G4 tower, I'd hook it up to my TV, and use it as a media center. Keep Movies, Music, and Pictures on it, and show them off on the TV that everyone uses. I'd wipe everything you wouldn't use any more to make space for media.
When connected to a TV, I would clear things like Adobe CS, Office, and all those other productivity programs that take up space and don't get used. Also, you don't have to wipe off Widgets, but at least remove them from the Dashboard so they aren't in the background all the time. Let the media run at its top potential, which should never stutter anyway.
It's a great streaming machine if the hard drive is small, too. Connect it to your TV, and avoid a reason to pay for a new

TV. If you're not a Nexflix or YouTube person, streaming from iTunes to iTunes will work just as well with this G4 as it would with an

TV. As long as you aren't desperate for the best quality possible.
Or...
If it's the iMac G4 all-in-one, I would put that bad boy in the kitchen in a heartbeat. Sit it in the corner of your counter space, and use it for the ultimate kitchen technology hub. Keep all your recipes on it, your future shopping lists of things you'll need, a wide array of great cooking applications, and especially, use it to browse
FoodNetwork.com, to watch and cook along with your favorite chefs, shows, and recipes.
Trust me, if you want a handy tool in the kitchen, and you really enjoy food and cooking, you could find a computer to be your best and favorite Sous-Chef. I just picked up a second G4 to put in my office at work, and over the Christmas break, I'll be thrilled to take it home and do this for myself.
The 15" screen is more than enough for me, for a use like this, plus, the smaller it is, the less room it takes up. I find that to be just the right size. Not huge, but a perfectly decent size. The way my kitchen is set up, the dome would sit in a corner of the counter top, and then, arcing the arm / tilting the screen up would let it sit at a perfect height / angle to fit under my hanging cabinets. I'd recommend that you keep in mind to not place it under a hanging can-opener, coffee maker, or anything with the potential to drip.
Some things to consider:
- Pick up the small aluminum USB keyboard without the number pad to minimize the space it takes up.
- Maybe a USB Mighty Mouse, so you can use right-click with one hand - or any other mouse with right-clicking. That way, you don't have to fumble with 2 potentially chicken-covered hands to right click something.
- Pop an Airport Card in it, to eliminate one more cable, minimizing clutter.
- If you're really tight on space, or don't have a place to put the Pro Speakers, you don't need them, as there's a speaker built in to the iMac's dome, that I find quite adequate on its own, unless you're a quality nut.
- Seran-Wrap the keyboard! It sounds crazy, but you'll inevitably find yourself with greasy, sticky, or messy hands, and need the next page of a recipe, or need to arrow down to read a few more lines. It's a keyboard cover that's cheap and disposable, and enough protection from most culinary arts.
I could go on and on and on about the iMac G4, and all the uses I have in my mind for it, but either way, I'd certainly find some use for a spare computer, even if it's almost turned into a luxury toy. At least it's getting used!

Plus, PPCs seem to run forever.
Holy crap, I had fun writing this!