I would think that the final ratio would have more to do with it than number of gears for hwy mileage.
Again, I said as 'a general rule'...if that was the case, why are almost all automakers currently producing 5, 6, 7 & even 8 speed automatics when they can simply 'tweak' the final drive ratio to an existing 4 speed automatic? One main 'long term' answer...better fuel economy & lower emissions.
One can also inquire what the EPA is doing when it comes to future regulations & how automakers have to respond in order to meet those requirements...but I would guarantee you that those changes would not include 4 speed automatics...
The number of gears in the box has turned into yet another dick swinging contest, just like tow capacity in pickups and horsepower in muscle cars. Once you get beyond 5 gears, you rapidly get into diminishing returns.
It's a trade-off between driveability and economy. A three speed could be made to deliver competitive fuel economy, but top gear would be a pretty big jump over second. Stick another gear in there to split the difference, much better performance, not much change in fuel economy. As you keep adding gears to the gaps, the marginal gains become smaller and smaller, while expense and complexity increase with every addition.
An argument for higher numbers of gears could be made if the power bands were very narrow as in diesel trucks (500-1000rpm) or peaky two stroke bikes, but modern engines are hugely flexible, and will spend very little time in the intermediate gears.
The following has two 1.8 litre motors (Corolla & Civic) and two 2.0 litre (Focus & Mazda3) motors. it also has two 4 speed automatics (Corolla & Focus) and two 5 speed automatics (Civic & Mazda3). No real trend for either set of cars.