[Yaristock]

You are technically correct that the four banger would be more efficient if we were to compare power ratios, but assuming that they both idle at the same rpm we wouldn't factor that in; because at idle(no accessories) what does that extra piston do other than create heat, and unused power?

edit: what you did above would be the same as saying a v8 is just as efficient as a i4 because at 2800rpm the v8 makes this much power when the i4 must be at 5000rpm to make the equivalent power.