ORIGINAL: B.L.E.
Another thing that needs to be understood is that the torque needed to turn a prop varies exponentially with the rpm. Double the rpm of a given prop and the torque needed to turn it quadruples. Because of this, it actually takes more torque to turn a 10x6 prop 15,000 rpm than it does to turn a 12x6 prop at 10,000 rpm. Not understanding this relationship leads to the common myth that four strokes make more torque than two strokes do. Without a tuned pipe, two strokes also have a broad power band.
The ironic thing is that as the horsepower race started with four strokes, fueled largely by the old F3A rules that allowed two strokes to be .61 cubic inches but allowed four strokes to be 1.2 cubic inches, four strokes became thirsty, and loud and really critical about prop sizes. Realizing that they have defeated the very purpose of four strokes, the rule has been changed to allow any engine or electric motor of any size as long as the plane stays under the 5 kilogram weight limit.
Here's a picture that shows just how quiet four strokes are. So quiet that mufflers aren't even needed.
Yes indeed, Your picture is the very definition of old time rock and roll. Now a days you can't sit behind that big "Quiet"
engine.
Anyway to topic: Every engine has it's place and aplication. If you are saying that 4C engines fly slow than you are probably seeing them in 3D planes which are built light for high angle of attack manouvers. Speed kills 3D planes so they are propped accordingly with low pitch large diameter props. Speed planes tend to be propped with high pitch smaller diameter props and relly on peek RPM to create thrust Hense a 2C is a better choice due to peek RPM. However if propped for it a 4C in the right airframe can "go fast" I have a Saito 100 in a H9 Spitfire with a MAS 15x6 and it moves pretty good. If I went to a 14x8 I could probably add another 10-15 MPH to it... but I like pulling power