Any current parallel ATA (IDE) hard drive will work in your machine. Don't get Serial ATA (SATA). Depending on the model of G4 you have, it may be limited to recognizing 128 Gb in size, so choose a 120 Gb hard drive. Mirror Disk Door G4's I believe can recognize higher than 128 Mb.
Otherwise older models can use software from InTech to support larger drives, or a hardware PCI-ATA interface card (both of which will increase your cost of course).
Look for a drive with a three year warranty (get it in writing). 7200 RPM is desirable, as is an 8 Mb cache instead of 2 Mb.
Thanks
Trevor
CanadaRAM.com