Treaty displacement and top weight make every armament change a design tradeoff. Replacing a Crown Colony-class cruiser's four-inch mounts with 4.7-inch dual-purpose weapons is estimated to add no more than about ten tons. Against Fiji's roughly 8,670-ton standard displacement, that increase deserves consideration without forcing the dramatic armor or machinery sacrifices imagined in the question. The same weight would matter more on a destroyer because it forms a much larger share of the ship. A weapon's absolute weight penalty must be judged against hull size and mounting position.
Notable Point · 1:01:07-1:04:12
Crown Colony 4.7-inch weight tradeoff
A treaty-cruiser answer puts the added weight of replacing four-inch guns with 4.7-inch dual-purpose mounts in proportion to a Crown Colony-class ship's full displacement.