Why the Town class did not need 4.5-inch secondary guns

Would the Town class have benefited from replacing its 4-inch secondary guns with 4.5-inch weapons?

No; the 4-inch guns were effective antiaircraft weapons, while the 6-inch battery already covered surface targets.

The Town class already divided its gun tasks between an effective 6-inch surface battery and dedicated 4-inch antiaircraft weapons. Replacing the latter with 4.5-inch dual-purpose guns would consume more magazine volume per round. It would also duplicate part of the surface role already handled by the main guns. The specialized 4-inch battery therefore suited this cruiser better than a larger dual-purpose replacement.