Apple made several models of the PowerMac G4. Outwardly, they were all the same until the Quicksilver models and the MDD or Mirrored Drive Doors models.
Go to this apple page on
how to Differentiate between G4 models. THis will tell you which G4 you have.
You need at LEAST the G4 "Digital Audio" to use the Apple Pro Speakers. The Digital Audio G4 was the last of the "Graphite" models. The Quicksilver and MDD models also could use the Apple Pro Speakers.
Apple Pro Speakers ARE NOT regular speakers.
This Article points out some differences between these and the G4 Cube speakers.
Bottom line: If you don't have a G4 Digital Audio or later model, you can't use these speakers (they don't work on a G5, either). No adapter will work. The G4 sends a digital stream to the speakers, not analog audio.
Also read
This article about sound from various Apple models.