I recently came across a couple of G4s that had dead logicboards. As I strip these computers and sell the parts, These were slated for the same fate. They would turn on but no video, no chimes the drives would not run. I took one just for fun and pulled the hard drive out of it and powered it up. I chimed the video came on and it went looking for the OS. I thought what a deal it just needs a hard drive. So I installed a known working hard drive and it went back to being hammer dead, unplug the drive and it would boot from the CD.
After a little checking I found the primary IDE connector to be bad. Well that either called for a add on ATA card or would it work if I connected the drive up with the Cd-Rom cable. What I wound up doing was setting the hard drive as a slave leaving the CD-rom master and inserting the hard drive in the carrier below the CD-Rom drive. The existing cables will work to hook up the drives and when you power up the G4 it will look for a boot drive via the CD-Rom then it will find the hard drive on the same bus and boot from it.
I set the CD-rom as the primary boot drive so that in case the Hard Drive failed I could still boot from the CD-Rom although I have found that you can set the Hard Drive as master and the CD-Rom as slave and it will still boot to the Cd-Rom drive if there is no OS on the hard drive. I have done this to two G4s one a G4 350Mhz and a G4 AGP 450 machine. Both are working with no problems. I am watching the Day after Tommorow on the G4 350 and as for dollars spent $10.00 for both G4s when I bought them at the yard sale.