Boring answer - Ebay the working parts. Dead computers aren't worth much on ebay. However, working individual used components are (as spare parts). The motherboard, memory, hard drive, monitor, and optical drive can be sold individually (if working) for more than one whole dead system. (The answer is boring because it's the first thing anyone will suggest.)
Artistic answer - Strip the box, and splay the electronic parts on the wall. It's cool to see what the inside of a hard drive looks like, or a disassembled display... if your interior designer is a Borg.
Hacky answer - Recover the display and use it to make an LCD HDTV projector. I've done this with a regular monitor, but I know people have done it with laptop monitors as well. This requires woodworking as well as electronics skills. http://www.lumenlab.com/forums/index.php?showforum=29 My username there is "AllThumbs" if you want to look at a completed project. It doesn't really require much woodworking or electronics skills, (hence, my username) but the more you have, the better the project will look (some of the other projects there are amazing.)
Hacky answer 2 - You could also turn the screen into a digital picture frame. Instructions for doing so with the G4 are located here http://www.instructables.com/id/Hanging-Laptop-Digital-Painting/step1/Remove-Hinges-free-the-display/
Instant storage answer -- Get an external USB drive enclosure and use the hard drive as an instant new drive for your new system. This idea may not be as useful due to the size and age of the drive, but external drive enclosures are cheap, so you might start with this drive and upgrade later.
Crafty answer - Recover the chips and turn them into little bugs. You can then sell them as a craft.
Humorous answer -- turn the optical drive into a cupholder. You hear the jokes about stupid people doing it. Do it for real as a joke.
Socially responsible answer -- Give it away. Find a charity that can use the parts. Or just post it on freecycle.org.
Environmentally responsible answer -- If you need to throw it away, don't. Make sure you recycle it. There are hazardous materials in any electronic item (with circuit boards and chips) that don't belong in a landfill. Costco, Dell, Office Depot, and many other companies will take old electronics off your hands without charge (and regardless of manufacturer) to properly recycle them. Alternatively, your local dump often offers electronics recycling days (to encourage people not to put them in the trash.)
The "How broken is broken" answer -- You didn't say how broken your G4 is. If it's no longer useable as a laptop, but still fires up and works, you could load it with linux, turn it into a firewall, make it into an MP3 media center, use it as a file or print server, or try out squid. (Squid is a web proxy server. You could use it to bypass inconvenient web filters, such as one at work or school. You proxy to the G4, and connect to the Internet from there, bypassing any web filters. Since it is a custom proxy server for only you, it will never make it onto a block list.)
Geeky socially responsible answer -- If it still runs as a computer, then you could donate CPU cycles to science. You may have heard of SETI @ Home, where they use idle CPUs to help search for extra-terrestrial life. The grid computing concept has advanced considerably since then. You can still do SETI @ Home, but you can also install a distributed computing framework and then donate cycles to one of many needy causes (such as finding a cure for cancer, solving engineering problems, performing calculations for climate change centers, etc.) http://www.distributedcomputing.info/platforms.html