Town-class gun balance at close visual range

Why did the early Town-class cruisers carry only two six-inch guns, and what was the effective range of their four-inch battery?

Their intended close visual-range fighting made the four-inch battery effective, with useful fire around 8,000 yards and an ideal range nearer 5,000 to 6,000 yards.

Two six-inch guns provided heavier individual hits, while the numerous four-inch weapons could do substantial damage at the short ranges expected when the ships were designed. The four-inch gun had a maximum range of roughly 11,600 yards at fifteen degrees elevation, but that figure exceeded the range at which repeated hits were likely. Nominal maximum range was therefore different from the practical fighting envelope.