There aren't standalone graphics drivers for the iMac G4. Any updates there might be are included in the system updates. Also, they are called "Kernel Extensions", or "kexts". Go to About This Mac, and see what version of 10.4 you are running. If it is 10.4.11, you can just continue to use this without having to do any sort of updates. If it is not 10.4.11, you must update it. You can try using the official Updater (your Mac has to be online for this, ofcourse), or you can download the 10.4.11 Combo Installer from Apple's website.