Although horsepower has its peak near 5000RPMS, in regular and even sporty driving on a stock engine, most TRs respond well to shifting between 4 and 4500.
In third gear, it's basically a dead heat with the 6cyl, weight, gearing (and wind resistance)whether you'll pull better between 4500 and 5000 or in 4th with that lower end torque of the engine coupled to the more direct 4th gear.