Are you trying to upgrade the OS to Leopard 10.5.x? If so the install DVD should look like this.
A gray installer from another model Mac won't work.
You mentioned iTunes. Are you trying to set up the G4 to sync one of Apple's iDevices. I have an iPad 2 and it will sync with Leopard and iTunes 10.6.3. However, all new iDevices require iTunes 11.x and an OS of 10.6 or higher. Since a PowerMac has a PowerPC processor, you can't install OS 10.6 or higher. You need a newer Mac with an Intel processor.