Any standard SATA interfaced optical drive should work. If you want a exact replacement I would look to ebay.
I am trying to confirm whether the G4 uses a SATA or SCSI interface on the drive.
This matches the GH41N number you provided. http://www.ebay.com/itm/Hitachi-LG-GH41N-16x-DVD-RW-DL-2-MB-Buffer-SATA-Optical-Drive-Black-/371641630143
but in my opinion the G4 is so old that I honestly do not think it would have a SATA interface, so I'd really like to be able to confirm it's interface before recommending a replacement.
The best way to ensure correct replacement is to remove the old part and get the actual part number off of it or just send a photo of the interface on the rear.