The iBook G4 has 256 Mb soldered on the motherboard and a single socket for a DDR SODIMM. If your machine is currently 256 Mb, then the socket will be empty. You can go up to a max. of 1.25 Gb with a single 1 Gb SODIMM.
Standard advice: order your RAM froim a reputable seller who tests and guarantees Mac compatibility with your model, offers a lifetime warranty and a no-cost return if it doesn't work.
Thanks
Trevor
CanadaRAM.com