Four bangers really aren't any more fuel efficient on the highway because they turn faster.
Not really. With identical gearing, why would a four cylinder spinning at 2200rpm at 100km/h turn faster than a V-6 at 2200rpm? Same speed.
Perhaps makers give four cylinder cars steeper gearing? Sometimes.
It mostly has to do with how many hp a given car needs to drive at a given speed, and how efficient the engine/gearing combination is at that speed. Aerodynamics are a big one, which is why even large family sedans typically get much better fuel economy on the highway than a comparable SUV with the same engine. A 3.5L Camry does better than the 3.5L Highlander thanks to the better aero profile and lower weight (less to lug up hills).
As for gearing, it plays a role, but taller gearing is not always better. An engine that operates at too low of an rpm may need constant downshifting. It may also mean larger throttle opening at lower rpm, which also hurts fuel economy.
A 1.8L equipped Corolla spins much quicker than many engines at highway speed, and yet is very fuel efficient. The small displacement, efficient engine (design and fuel management) and slippery body are the keys. The shorter gearing is suited to the engine's power curve.
Fuel economy is much more complex than "turns faster."