That depends on your computer, not your OS. It doesn't matter if you're running OS 9 or OS X, the limit depends on the drive controller in your computer. So, which model G4 do you have? If it is a graphite tower, it has a maximum limit of 137 GB. If it's an early Quicksilver tower (733, 867, or dual processor 800 MHz), it also has the 137 GB limit. However, if it's a later Quicksilver tower (single 800, 933, dual processor 1 GHz) or a mirror drive door tower, the sky's the limit.
Wait, do you have a G4 or a G3? You originally said G4 and then you said G3.