On the G4 Mac open the Terminal app (in Applications -> Utilities) and type the command machine. If you feel unsure what type of PowerPC processor your Mac has, go to the Apple menu and choose 'About this Mac' and check the processor field. It comes in different tuned versions for specific PowerPC processor families: TenFourFox is a fast, efficient Web browser always based on the current version of Firefox, but unlike the main distribution TenFourFox is compatible with OS X 10.4 and 10.5 running on PowerPC processors.
