The following assumes that your optical drive in the G4 is a combo CD/DVD drive. If it is not, there is your answer.
Which OS 9? OS 9.1 will not start up your Mac after you have installed OS 10.4.x. If the new 9GB drive has not yet been formatted, with OS 9 drivers installed, and if you did not set up the 30GB drive with OS 9 drivers (from within Disk Utility under Tiger, or from an OS 9.2.1 CD), you will also not be able to access either drive from an OS 9.2.1 Full Install CD.
Use Disk Utility::Erase, with Install OS 9 Drivers checked, to format the 9GB drive from the 30GB drive. You should then be able to boot from an OS 9.2.1 Full Install (white with amber 9) or a grey G4-specific CD with OS X/OS 9. If the latter has only OS 9.1, you will need to download and install the 9.2.1 and 9.2.2 updates from Apple, especially if you wish to use OS 9 in Classic Mode within OS 10.4.x.
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