A fast processor (G4), now obsolete because of the GeeFive (G5).
Has been used in the GeeFourCube, GeeFourPowerBook (Titanium and Aluminum incarnations), GeeFourPowerMac, GeeFourXserve, GeeFouriMac, GeeFoureMac, and now, the GeeFouriBook.
One might note that in the technical parlance, what Apple refers to as the G4 is actually the PPC74xx. The first was the 7400, and I think they’ve made it up to 7455, and possibly beyond. One should note that it’s neither that fast, nor obsolete, though.