reset older iMac
Completely erase and reset iMac PowerPC G4, Mac OS X (10.3.9)
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Hi,
Which exact iMac G4 model/year?
https://everymac.com/systems/apple/imac/index-imac.html
You would need an appropriate system disc if you wish to erase/wipe the hard disk and reinstall the operating system.
If you just want to erase the hard disk, there may be other solutions.
Do you have access to another Mac with FireWire?