Why some cruisers carried four-inch guns

Why did so many cruisers carry four-inch guns?

They were useful against merchant ships and could also hurt many lightly armoured protected cruisers of the period.

Four-inch cruiser guns make sense in their pre-First World War setting. Merchant ships were an important expected target, so rate of fire and a useful shell mattered more than the ability to penetrate battleship armour. Many protected cruisers also carried little armour over much of their structure. A four-inch battery could deliver a large volume of effective fire against the opponents these cruisers were likely to meet.