Install an OS on Powerbook G4 Using USB drive

If you can tell us what OS X version you have we can tell you if it can run on that Mac. You can normally 'restore' a bootable OS X installer onto a Firewire disk - that allows you to boot to install or repair the system.

Another option is to find a PowerPC version of a Linux distro. That will allow you to use the Mac without owning a copy of OS X.

I hear good things about Ubuntu Mate on PPC Macs, see their forums if you want to install that.

Another option is to try to repair the Mac via Target disk mode - do you have another Mac available to use for that?

Share files between two computers with target disk mode - Apple Support

A flashing question mark indicates the OS cannot be found - hold 'alt' (a.k.a option) at startup & see if any disks are listed as suitable for booting from.

Also reset the PRAM…

How to reset NVRAM on your Mac - Apple Support

It may cause the system it seek out other boot disks but it won't fix anything.

Most G4 models will not boot to anything other than the internal drives or an external drive connected by Firewire, i.e. no USB booting. In addition no installers were created to run from anything other than optical discs. Essentially G4s were only set up to have OSX installed using CD or DVD installer discs. G4s can only boot to versions of OSX prior to 10.6. Do you have installer discs?

What OS are you using now? What OS do you want to install?

Your very old machine can't install any OS X version you may have downloaded.

10.5 could be the last version of OS X you could install but that would only be via an Apple DVD of that version.

Apple no longer sells those old versions but many third party resellers do.

Find one starting with some searches.

Be ready to pay for it. In all likelihood it will cost you more then the PowerBook is worth.