This is probably more a question, than an answer, so comments are welcome...
You refer to an angle of attack of 4 degrees being optimum, and the obvious requirement for more power with more load. As the power is increased, the rotor RPM is not. So, it must be that the increased power is being consumed by an increased blade angle (hence the need for more pedal).
So we agree that an increase in blade angle is required, to produce greater induced airflow, to carry a greater load, but what is the reference for that blade angle, the plane of rotation, or the angle of attack relative to the oncoming air?
Perhaps the blade angle is measured relative to the oncoming air, which is moving faster becasue of the increased power, to create thrust to oppose weight. If the oncoming air is moving faster, the blade angle could be the same relative to the oncoming air. Climbing or descending would also be a factor in this.
I'm sure that someone wiser than I can support this theory, or help me understand better too...
In the mean time, I better go back to my books...
Pilot DAR