Older PowerPC Macs, like your G4 PB, won't boot from a USB drive. However, there is an alternative way.
Installing OS X 10.4 'Tiger' on DVD-Challenged Macs Using FireWire Target Disk Mode http://lowendmac.com/misc/06/0710.html
Installing Tiger Without a DVD Drive
How To: Installing Tiger Using FireWire Target Disk Mode
Title says it all, I got a powerbook g4 that has a faulty optical drive
Might as well try the cheap, low-tech approach before jumping through a lot of hoops. You'd be surprised how may apparently dead optical drives can be resurrected by a simple drive cleaning disk. They cost from US$5-15 and can be bought at most electronics or office supply stores that carry computer gear and supplies.
Very recently, a friend had an older Macbook Pro with a non-working optical drive. The local Apple Authorized Service Provider checked the computer and said the drive must be replaced. To make a long story short, I loaned my friend my drive cleaning disk and his drive now works perfectly