The message that you received when you attempted to install Cubase told you that you did not have a new enough version of OS 9 installed on your G4. It does not matter which version of OS-X you have installed on the G4, Cubase is going to run in the Classic mode and needs to have the proper version of OS 9 for it to install and run. In other words, get the upgrade installer for OS 9.1 or higher and install it. Then, Cubase should install without any more hesitations.