I'm using a G4 iBook also, and when the battery is fully charged it is saying i have about 4 hours to go.
This however drops faster than times goes by and i would say i have something inbetween 2,5 hours and 3 hours of battery time, and probably depending on how i use the computer.
I do not know of any way of resetting a battery, gosh that would actually be THE invention on batteries these days.
There is one thing though that you can do, and not just with the battery on the iBook but with every battery you use and feel it's giving lesser power than should.
Drain your battery completely, let the machine run until it really shuts down from itself (off course, don't forget to save your work).
After that, completely charge the battery, and i mean completely, give it long enough even when the icon already say it's fully charged.
Then... i would say do this again, drain it completely and charge it again.
Most people tend to use batteries for half or 75% and then charge them, just to be sure.
This is ok but this is not really well for the battery itself.
The modern batteries might be handling this better but i'm not sure honestly.
Hope it works, maybe others have different ideas and knowledge.