If either engine can turn the same prop the same rpm, they have the same torque.

But 4C engines don't turn up the rpm that a 2C does, so they are propped different.

The speed that a model can achieve is proportional to the power output of the engine (which varies with the operating rpm), but it's not a linear relationship. The relationship of speed to power is (power ratio)^.333

It takes 8 times the power to double the speed.