If your G4 iBook originally shipped with Panther, and you upgraded to Tiger, then it makes sense that you could go back to Panther.
If your iBook G4 shipped with Tiger, then it is highly unlikely that you could install Panther on it. Highly unlikely, though, could mean yes, it's possible.
I am pretty damn sure that if you purchase a MacBook that ships with Leopard on it now, that you could not install Tiger on it (but, as I said before, there is a possibility it would work -- I wouldn't bet on it, though).