Why retain a mixed gun battery?

If the 4.7-inch guns become dual-purpose, why keep the twin four-inch mounts?

The four-inch guns provide proven high-angle fire while the modified 4.7-inch mount is still unproven, and a mixed main and secondary battery fits contemporary cruiser practice.

The four-inch guns are already available and have an established high-angle role. The modified 4.7-inch mounts still need to prove that they can perform as intended. Keeping two twin four-inch mounts therefore gives the ship an AA battery from the outset. The mixed armament also follows the preferred cruiser-style division between a main and secondary battery.