Be careful about this one,
4000 Rpm with normal final gear ratio should give you approx 120 Kph. that is fast enough for me.
Changing the final gear ratio to give a lower Rpm whilst maintaining that 120 Kph. will only act as an overdrive, and put extra loading onto the engine, this will only necessitate a more aggressive and a more open throttle position in order to maintain your road speed. IE. imagine a steep hill and opening the throttle in order to maintain the required 120 Kph.up that gradient.
An engine operated under a heavier loading, in order to maintain the required torque output to maintain the required 120Kph in my opinion will require a considerably more open throttle position and consequently more fuel than as in normal use.
Hope this makes sense.
Cheers,
Winston66.
Northmpton