Why use triple four-inch guns on battlecruisers?

Why do you keep using triple four-inch guns, and why were battlecruisers not fitted with larger secondary guns earlier?

Four-inch guns could fire rapidly enough to stop small, fast torpedo craft. Larger secondaries made sense once navies expected heavier destroyers or cruisers and accepted the extra weight and complexity.

Secondary calibre was a doctrinal choice rather than a simple measure of firepower. A dreadnought's main battery could fight capital ships, leaving its secondaries to stop destroyers and torpedo boats that broke through the screen. Multiple four-inch barrels supplied the volume of fire needed against small, manoeuvring attackers before they launched torpedoes. Six-inch guns became more attractive against larger destroyers and light cruisers, and they were useful for commerce-raiding duties. Mount layout, dual-purpose anti-aircraft demands, and confidence in fleet escorts also changed the calculation. The UAD testbed brings these trade-offs together: a secondary battery records the kind of battle its navy expected.