The four-inch, 4.5-inch, and 4.7-inch roles were not inevitable

Were the four-inch gun, 4.7-inch gun, and 4.5-inch gun naturally optimized for anti-aircraft, surface, and compromise roles?

No. Their apparent roles emerged from programme history: the 4.5-inch began as a four-inch replacement, while the 4.7-inch remained low-angle until wartime exposed the need for a destroyer high-angle weapon.

Each calibre's wartime role emerged from programme history rather than a technically ideal niche. The 4.5-inch system started as a replacement for the four-inch anti-aircraft gun and then expanded into a dual-purpose programme. The established 4.7-inch weapon remained acceptable for surface fire and was not developed for steep angles until the operational requirement became urgent. By then neither a 4.7-inch conversion nor the 4.5-inch upper-deck mount was ready, so destroyers received four-inch substitutions instead.