You'll need to install a SATA PCI card in the G4 Mac. You can either buy a PC one on eBay or somewhere else and flash it to Mac using a PC or you can buy a Sonnet or some other type of Mac SATA PCI card.
Generally, the Sonnet or other Mac SATA PCI cards run around $80. Buying a PC card and flashing it yourself will cost about $10 and a bit of your time and expertise. If you buy the PC card and flash it usually it doesn't come with cables. So, you'll need to purchase the SATA data cables and Molex to SATA power lead splitter, or a Molex to Molex cable if your SATA drive supports Molex power.
As Intell has said, G4 PowerMacs are PATA (IDE).