Charles, the "best" prop pitch is another of the parameters that is meaningless if taken out of the context of the other parameters. The qualities you attribute to "4" only apply to certain combinations of airspeed, altitude, Kv and voltage (and therefore RPM), etc.. For example, if you installed a motor with twice the Kv (all other parameters unchanged), you might have a problem with a stalled prop and/or overloaded motor at low speeds. OTOH, a motor with half the KV on that same pitch = 4" prop might not pull enough watts to taxi well, much less actually take off and fly.
The reason you like 4" pitch is because the particular combinations of motor, battery and airplane you commonly use just happen to work well with that pitch. Coincidence, nothing more.
I've found that pitch speed is a good starting point, provided that you allow for the effects of prop efficiency. Take the plane's airspeed at the operating point you want to design for, then divide by about 0.70 to 0.80 for cruise, 0.60 to 0.70 for takeoff and climb. The speed you get is a good starting point for pitch speed.
For example, if you want to cruise at 40 MPH, divide that by about 0.75, and you get a pitch speed of about 53 MPH.
We can convert that to feet per second if we multiply by 22, then divide by 15, which gives us a pitch speed of 78 feet per second. Multipy that by 12 and we get 939 inches per second.
If our motor runs at 5000 RPM at cruise power, divide by 60 and we get 83 revolutions per second.
Divide our pitch speed in inches per second by the revs per second, (939 / 83) = about 11 inches pitch. If our motor had twice the KV and ran twice as fast (10,000 RPM), we would need a pitch of about 5.5 inches.
Note that each of these examples is appropriate for their operating characteristics (airspeed and RPM in particular), and that neither is equal to 4.
