Apple has a list on its website which tells which machines get Panther for "free." If the machine was packaged at Apple in the last 10 days or so then Panther would be inside the box. If its been in the store for a while or was shipped out from Apple more than about or so ago then you have to pay $20 for the upgrade. In general, though, if the G4 machine is less than 1ghz, even if new, you won't get anything.
Sorry, I can't locate the link on Apple's site right now--I was just looking at it last night.