Your engine is operating at its peak efficiency during full throttle operation. That is because of the low pumping loss, and more importantly, the increased density of the mixture during combustion.
But peak engine efficiency (the ratio of power developed to fuel consumption), does not mean peak MPG. MPG is much lower during full throttle.
Can your vehicle get better MPG when you downshift from 4th to 3rd? Perhaps, but there are too many variables, including, increased engine friction in 3rd, slightly increased pumping losses (though that would be minor for this comparison), and most important, the manufacturer’s programming for timing and fuel mixtures at those different rpms.