I have never heard of this issue (even on the original Steinberg Forum) so please give me a little more info.
A) What exact version are you loading (5.1r1, 5.2 pb, etc.)
B) System info (G3, G4 Digital Audio, G4 Quick Silver, G4 Mirrored Drive Door)
This is the way I load Cubase....maybe it will help
1) Get a Fresh 9.2.2 Install Going (see the Mac OS 9 Lives website if you need Help)
2) Run the Cubase 5.1r1 install via the last rev from the Steinberg website (do Not load the CD Version, you will need to have the CD in during the install and it will authorize.... no dongles, USB keys, or web authorization needed)... DO NOT Open Cubase yet after install
3) Turn of Appletalk (via the chooser)
4) Turn off virtual memory
5) Load all MIDI and sound card extensions
6) Install OMS 2.3.8 (if needed for MIDI interface) or FreeeMIDI with OMS Support
7) Set Cubase Memory to 300,000 or 400,000 via "Get Info" CMD-I and Memory (You should have at least 512MB to 1 GB in your G4)... do not load more than 1 GB or the system may act less stable... IMO Mac OS 9 runs the most stable on exactly 1 GB... not more
Launch Cubase and pick OMS In/Out
9) Load all Virtual instruments and Effects and Organized them... Under VSTPlugins you can make as many category folders as you want (EQs, Delays, Reverbs, Drum Machines, Synths, etc.) and drag all related plugins to the category... this makes it so easy when loading / looking for instruments and effects