There were sub-models of the G4 iMac and they do not all use the same RAM. If you gather some extra info, we can better help you.
First, what is the approx screen size measured diagonally?
Second, do "About this Mac" from the Apple menu and, in the resulting window, click the "More Info..." button. That launches System Profiler. From the opening Profiler screen, please tell us wha the second line under "Hardware overview" reads. It should be labled "Machine Model" in OS10.4 and is followed by a code such as "PowerMac6,1" That code and your screen size with tell us your exact sub-model.
While in Profiler, do one more thing. Form the left-hand "Contents" pane, click "Memory". Looks like this:
except yours will show only two slots. Copy/paste the info under "Memory slots" into your reply; that tells us what slots are now in use.
This takes less time than it does to read and will allow people to give you very targeted upgrade options.
Also post your approx geographic locations and we may be able to point you towards some trusted RAM vendors.