I use macs at work, being an Apple tech and all
What I would do is run a disk repair util on the drive like Disk Warrior or Nortons Disk Docotor. They can be booted from the CDs they came on on your G4 system. See what they have to say about the disk. If you dont have one of those apps connect the drive to a machine that has a good working copy os OS X on it and use the Repair Disk function from Disk Utility on it and see what that does.
If I was to make a wild guess at whats going on I would have to say the the hard drive is failing.
