Low mounting weight eased the twin four-inch upgrade

Why was it comparatively easy to replace a single four-inch Mark V mounting with a twin four-inch Mark XVI installation?

Because four-inch guns and their mountings did not weigh enough to make the twin installation prohibitively demanding.

Weight was the central reason the replacement was manageable. Four-inch weapons and their mountings did not impose the penalty associated with much heavier naval artillery. The barrel count could therefore increase without demanding a proportionate reconstruction of the ship. A weapon's absolute size can create modernization options that a simple comparison of one gun against two might obscure.