Forgive me if I am stating the obvious. At first I didn't understand where the 488 GTB got its name. So in the case of the 458, my understanding that 4.5 refers to total discplacement and 8 refers to number of cylinders. In the case of the 488, 488 refers to the capacity of a single cylinder not the total discplacement of 3.9 liters. The GTB, Grand Touring Berlinetta, pays homage to the way the 1960-70 Mid engine V8s were named. I'm glad to see this tradition of nomenclature continue --using numbers which reference some aspect of the engine and mechanical traits of a new model.
Its the size of each cyclinder. If you divide the size by 8.... Well that's what they said at the presentation.
It harkens back to the old days when Ferrari routinely designated its cars by cylinder displacement. So the 250 GTO was a 3 Liter V12 and the 500 Mondial was (believe it or not) a 2 Liter 4 cylinder. We can assume that Ferrari did not want to designate the new V8 by a lower number which it would have if it continued the total displacement tradition. So they went back to the cylinder designation. By this metric the 4499 cc 458 is the 562 Italia. PS and (can't resist it) the F12 is the 522 Berlinetta and the FF is the 522 Ferrari for Four.