Is this a used PowerBook G4 you bought? If so, the previous owner should have given you all the discs that originally came with it, and one of them (if more than one) would have Mac OS 9.0 or 9.04 on it. These would be grey discs labeled especially for that specific model; any other discs will not work, with the exception of a Retail Mac OS 9 disc, which isn't sold anymore.
What year is this PowerBook G4 from, and did it come with Mac OS X installed? Also, are you in the U.S. or another country?
You may be able to purchase a replacement set of discs for this model, but you'd have to call Apple Phone Support to find out, and you will need the serial number of your PowerBook when you call. You can find that by clicking on the Apple menu at the top left of the Finder window and choosing About This Mac, and then from the window that appears, click on More Information… That will open System Profiler and you can find your serial number listed under Contents > Hardware; it should be the last item in the list for that window.
In the U.S. call 1-800-275-2273; elsewhere in the world, refer to this document: https://support.apple.com/kb/HE57
Only four PB G4 models can boot using OS 9 -
None of them can use a retail OS 9.1 or earlier install CD.
Two which originally came with a model specific version of OS 9.1 can use a retail OS 9.2.1 Install CD - the model named PowerBook G4 and the one named PowerBook G4 (DVI).
The other two need the original disk set which came with them in order to install a bootable OS 9.2.1 or OS 9.2.2 version.
If you're not sure which model you have -