The 4560 and 4550 type cabs, front loaded horns, lived some years longer than 4520 in the JBL catalog (in 1986 they're still there in 46XX series). Most probably because they were long throw units, therefore having increased usefulness for shows, touring, theater, particularly for the various versions they made of the 4560. The latter was more manageable on the road, even stacking some for increased output.

As for the 4550, at 195 lb empty and a net bass-reflex enclosure volume of 20 cu.ft.!, size and weight (loaded) made it sort of a monster to carry left and right... (btw 4520: 215 lb empty).

Richard