Why early cruisers mixed four- and six-inch guns

Why did Bristol-era cruisers use mixed four- and six-inch batteries?

The six-inch gun served against cruisers while the quicker four-inch gun was better against destroyers; later rapid-fire six-inch weapons reduced the compromise.

The arrangement reflected two different target sets. Six-inch weapons were preferable against other cruisers. Faster-firing four-inch guns were more suitable for engaging destroyers. Uniform six-inch batteries became more attractive once improved mountings increased their rate of fire.