What model G4?
How much memory is installed?
Have you installed memory recently?
Have you run Repair Permissions or Repair Disk? Or any maintenance on the machine?
Is it a new install? Tiger indexes the drives for Spotlight when first installed. If that's the case, just let it do it's thing - for whatever time it takes - up to 30 minutes for a big drive, more if you are indexing extrnal drives also.