The whole 'G' business isn't Motorola's or IBM's idea anyway. The G4 is really a G3. The core didn't change like it did from G1 to G2 to G3.
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Wrong, the G3 is based on the 603 which was aimed at low power/value market, while the G4 is based on the 604 and 601, but gaining the 603's power management. Apart from the altivec unti, the main difference between a G3 and a G4 is the FP Unit. The G3 has only a single precision FPU, but emulate double precision, while the G4 has the better FPU from the 601/604.