A fifth Iron Duke adds firepower and treaty leverage

Where would a fifth Iron Duke-class battleship matter most?

It would probably add another battleship at Jutland and later provide Britain with extra tonnage to exchange in treaty negotiations.

With Emperor of India absent for routine servicing, a fifth ship could leave four Iron Dukes available at Jutland. That adds another set of 13.5-inch guns to a British line that already enjoyed a major numerical advantage. One ship does not guarantee a different outcome because its position, crew, and hits remain contingent. After the war, the extra hull also becomes bargaining mass that Britain can offer for permission to build newer capital ships. The same ship therefore affects immediate battle strength and the later politics of replacement tonnage.