answer:Well first off take a look at these two links . It looks like G4s are not very easy to replace the hard drive yourself, and I;m guessing if you are asking this question you would not be comfortable going through the trouble of that yourself. Check out this post and maybe you can find more info starting there. As far as just hard drive selection. You need to be looking for 2.5inch drive, at least 5400 RPM and ATA, not SATA. I believe for the G4s.