The way I see it, a 4cy is a fully pumped engine, the entire length of the stroke is used to pull fuel into the cylinder, and the entire length participates in the combustion downstroke. They get more power per "pop" useable. Another big plus is that the power range is wider because the RPM range is smaller. Thus the larger, higher pitched props are used. Anytime you can turn a prop at a lower RPM, and still be in the power range of the engine, you have less drag on the individual prop blades for each lb of thrust produced. Even if a prop had zero pitch, there is a drag involed in pulling something with a non zero thickness through the air. This drag increases exponentially with speed through the air. This makes a slower turning prop more efficient.