ChuckD, the main diff as it related to our planes is that 4S can turn larger props compared to a similar powered 2S. This is because 4S engines produce more torque at lower RPMs compared to a 2S. Take two 1HP engines. The 4S may produce the 1HP at 9KRPM, while the 2S may produce the 1HP at 14K RPM... both are 1HP engines. The 4S may have a 14x6 prop while the 2S would have a 10x5 prop (just as an example as the listed props work out to about 1HP at the example RPMs.) The advantage then is that 4S with the right propeller will product more static and low end thrust becasue they can swing larger diameter props, but speed suffers. Likewise, the 2S with the right prop will have poor thrust, but can produce higher speeds. In the above example, my Thrust calc is showing the 4S w/14x6 @9K produces 9.3lbs of thrust and has a top speed of 50MPH, and the 2S would produce 6.7lbs of thrust and a top speed of 70MPH. You can play games with props, like putting a very high pitch prop on a 4S to gain speed and putting a large very low pitch prop on a 2S to gain thrust. But IMO, you are better off matching the engine to your purpose. So, which engine offers the most advantage really depends on what plane we are talking about, or maybe more importantly, what type of performace you want out of the plane.