The iBooks spec is nothing compared with the iMacG4s, you have to remember they still sell the G3 iMac and that is $200 cheaper (If I remember correctly) that the iBook.
iBook has a G3 chip, new iMac has the G4 chip. iBooks are either 500MHz or 600Mhz, iMac is 700MHz and 800MHz. iBook comes with either a 12.1" or 14.1" LCD, the iMac has a 15"... You can find all the differences at Apple's web site, just click on hardware and start comparing. Also look at the optical drives in either computer...