Because of the vintage iBook G4 is not considered to be supported, an official Apple Store
would probably not be of much help in diagnostic, repair, or replacement software, etc. If
you are in the US, it still may (or not, depending on who you could talk to) be able to get
a replacement copy of the correct OS X full install DVD from Apple main support. Some
persons have been known to get a copy (white label) for a fee, after calling the 800# for
Apple and ask for OS X system specialist, and state the computer serial number, and
ask if you can buy a replacement copy of Tiger 10.4 (or if the computer shipped with that
and you don't know for sure if their replacement 10.4 DVD would work, ask if you could
order a replacement copy of Leopard 10.5 DVD, too. There may be a fee, for each.
If you are not in the US, you may not be able to get a replacement copy (white label) for
either of these; depending on the build series, (access info by use of serial number) and
see which OS X it shipped with, to know what it can use, if the iBook G4 has a processor
faster than 867MHz it could run Leopard 10.5 (retail install DVD) and that is no longer for
sale new; you could hope to find a good copy in the used re-sale market online or in an
older stock that some computer repair shop may still have. A DVD from another model
Mac will not work. A download from the internet of an OS X system, is illegal, ill-advised,
and usually will not function. Once installed, there still are update steps for download.
The correct original grey install-restore DVD disc set, had part numbers for the specific
computer model and build series; so they should be kept with the intended Mac for life.
Sorry to not be of much more help in this matter.
Good luck! 😐