Q: ibook g4 battery is bad.

Apple battery replacement service for iOS devices and notebooks - http://www.apple.com/batteries/replacements.html  - read at the very bottom for older notebooks.

Have you tried resetting the PMU?

http://support.apple.com/kb/HT1431?viewlocale=en_US

If that doesn't help, choose either the 12-inch or 14-inch battery (depending on the size of the iBook) here:

http://eshop.macsales.com/shop/Apple/Laptop/Batteries#ib

No?

"If you own an older model PowerBook or iBook, several third parties offer compatible batteries, and Apple authorized service centers can order batteries for models up to seven years old."

I think what Limnos is referring to is this at the bottom of the last paragraph

" If you own an older model PowerBook or iBook, several third parties offer compatible batteries, and Apple authorized service centers can order batteries for models up to seven years old."

Sadly your not get much life out of your current battery, unless it's a replacements that is not working correctly.

You would need to order a replacement. Ronda link of the third party solution seems to be a good choice

Then go for the new battery.