The HD from the G4 is the same physical size as the ones that go into the PM G4.
It will almost certainly boot from that drive, but just in case, you can put the iMac drive in as the second drive and boot from the G4 original. (assuming that there was one with a valid system on it)
The power supply on my wife's old iMac G4 died a few weeks ago and I recently replaced the computer with a Power Mac G4 I bought from ebay. I have "gutted" the iMac and want to know if I can stick the hard drive from the iMac into the Power Mac and be able to access the data. Thanks for any help.