Compatible CDRW drive?

You can use any external CD/DVD optical drive with your G4

Here is a good source of Mac hardware that will have what you need.

Forget using a Blu-ray drive.

http://eshop.macsales.com/item/OWC/VLU2SD24XS/

http://eshop.macsales.com/item/OWC/MR3F82SD24S/

Here is a solution that combines a CD/DVD optical with an external hard drive, SD card reader and Powered USB hub.

http://eshop.macsales.com/item/NewerTech/MSMXDW1.0TB7/

Note, if you have a PCI PowerMac G4, it won't be able to boot any external drives in Mac OS X.

AGP though you can at least boot Firewire drives in Mac OS X.

http://support.apple.com/kb/HT3082