Q: What type of battery fits a PowerMac G4

Your screen size and Powerbook model determine the battery type, but they are a dedicated Apple design--you can't buy them at the hardware store.

This firm has PowerBook replacement batteries:

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

They are the most trustworthy of the battery vendors. They sell a third-party battery because Apple no longer sells PowerBook batteries.

A friend has bought very good batteries for the older PowerBook G3 from this firm:

http://www.global-batteries.com/index.php/cPath/1_12/sort/2a/page/1

I have no idea if their G4 batteries are as good as their G3 batteries, which worked quite well.

I'll second the recommendation for NuPower batteries from OWC.  They meet or exceed the OEM specifications and work well.  I've used them in my old G4 powerbook and there is one now in my late-2008 macbook pro I use everyday.

Ignore my last post. My apologies!

I had lurking in the PowerBOOK forums and and did not notice that you asked about a PowerMAC So my answer wrongly assumed you had a laptop Mac.

The battery for a G4 PowerMac is usually this one:

3.6v Newer Technology Lithium 1/2 AA PRAM Computer Clock Battery

but you should post more details about yours so we know the exact model. There are a lot of PowerMac G4 variants and some people ask here about G4 iMacs, too.