Unable to boot into 10.4 installation disc
Most PowerMacs can't boot off of USB drives natively. They can boot off of a FireWire drive, however. If you have an external FireWire hard disk handy, you could try using Disk Utility to clone the 10.4 boot disk onto the drive and booting off of that. There is also (supposedly) a way to make the system boot off of an external USB disk, but it involves editing an nvram variable. This article explains how to do that.