Reboot. Hold down CMD+V. This is the command to show a verbose boot. You'll see a bunch of white code on a black screen. What we want to get a sense of is what errors are being thrown up or where the boot hangs. So, let us know what the readout says (particularly the last few lines where the boot hangs).
Using verbose mode can tell us this (more or less).
Put your PPC G4 in Target Disk Mode and using FW access the hard drive from any other mac that has an FW port. Get all your work data and files that way you are safe to do whatever you want on your G4.
Press "T" on the keyboard and hold it after you push the powerbutton to turn on your PPC G4. Release it once it presents a blue screen with a firewire image. This will make your PPC G4 to be recognized as a hard drive from your other mac thru FW.
I think Macbook Air does not have any firewire interface.