a PCI SATA card which will give you the necessary connections to install that drive.
Be aware that some of the G4 models, and you did not mention which one you had, have an HD size limitation on them.
The PCI and AGP graphics machines, and possibly a few others, are limited to 127GB drive size. It does not matter what size drive you put in there, it will only register 127GB.
With the PCI SATA card that limitation will not apply.
I have a G4 Mac I wanted to buy a much larger drive. I bought a 1TB SATA internal drive, but when I tried to install it I found out it could not be installed on my G4. The motherboard does not have slots for the cable that came with the SATA drive. The hard drive that came with the computer is Ultra ATA from Seagate 180 GB. I found out that ATA and SATA have completely different configurations. I am planning to return it to the store. Is there anyway I can use the SATA on my G4, or should I just buy a larger Ultra ATA? Happy Thanksgiving.