2S vs 4S for 3D: The real issue with 3D is getting close to a 2:1 thrust to weight ratio and having controllable linear power. As long as you can turn a suitable prop and you have linear throttle response, I really don't see what 2S vs 4S matters.

Real world, small 2S engines like to wrap up to high RPMs, so it is difficult to prop in large diameters. A small 4S on the other had produces more torque at lower RPMs, which is well suited to larger diameter props. For smaller planes, the 4S seem to be well suited to meeting the prop requirements for 3D.

As you get larger (35%/40%), the twin 2S engines produce better low end torque not to mention they are gobs lighter than a 4S would be at that size. In that range 2S seem to be the best choice.

I'd say the cross over point seems to be about 1 to 1.5 cu inch. 2S in that range start producing better low end torque and rev to lower RPMs like their 4S counter parts.