There are some do-it-yourself (DIY) online repair guides that may be of help
should you try & replace the internal hard disk drive or other components.
You would need to have the original OS X install-restore DVD for the OS X
version you have, to re-install a system; unless you had a working clone
in an external hard disk drive. In any event, the correct disk utilities for
setting up the new hard disk drive are on the bootable system DVD.
With the DVD in the optical drive, you can hold the C key on startup.
The computer will try to locate a startup disc, and should find the DVD.
However these portable models can be rather tedious to attempt a repair.
And without knowing the extent of the issue or failed component, results
may not be forthcoming. Also, some attempts to DIY repair have been a
cause of other damages resulting in harder to diagnose problems.
This site offers a few repair guides for certain models of Apple portable:
•Mac Laptop Repair Guides - iFixit: (in 5 categories of model series)
https://www.ifixit.com/Device/Mac_Laptop
There are 9 categories of PowerBook (only two with processor G4)
https://www.ifixit.com/Device/PowerBook
So perhaps your model is the one shown here; if not, identify yours before
attempting to use the info, and match the correct guide w/ your PB/G4.
•If yours is a 12"/15"/17" PowerBook aluminum 1.5GHz, this may be one:
https://www.ifixit.com/Device/PowerBook_G4_Aluminum_Series
Some other information may be found at powerbookmedic.com where some
repair info and links to possible part numbers appear in their retail site. For
parts that are no longer available, you may have a chore in finding right ones.
You could try & get a working PowerPC Apple portable already repaired and
try to access your projects and saved files from the problematic PowerBook.
Older repaired models are sometimes available from specialists who repair
them for a living; some of these are quite good at it, and may have older
vintage models for sale. Or they may be able to fix yours. The better ones
likely are not near you, so may require shipping the computer away to fix it.
wegenermedia.com is a fair site to look to, they fix computers, have parts
and that is what they do. Some models of older Macs appear for sale, too.
The PowerBook G4 computer would use an ATA/IDE or PATA (parallel ATA)
kind of drive and not to physically large. You could see what is offered by
resellers of Apple oriented replacement or upgrade parts. A 2.5" drive, if not
too thick, and of correct power spec, may be a match.
However if you do replace the hard disk drive, you'd have to re-install all of
the software, and the content of the old hard disk drive may be lost. A good
Apple trained service provider may be able to find what is really wrong, and
if it is the hard disk drive, save content from old one for you, to access after
the computer is running correctly again. A backup copy or clone could help.
If you have technical skills, and know how to replace tediously small parts in
portable computers, then this may not be impossible. If not, then you should
find someone who has experience in troubleshooting, to help back up what
ever can be from your current computer HDD, to some external device, and
then proceed with the repairs -- whatever they may be.
Could be the logic board in your computer may be at fault, if so a hard drive
by and of itself, would not resolve the issues as presented by the machine.
So hopefully you can locate someone with computer repair skills at a local
university or college, to help troubleshoot & repair your PowerBook G4.
Good luck & happy computing! 🙂