Hello Jim,
1) Is resistor R4 in the circuit permanently, or only switched in while adjusting R3?
Yes, R4 is permanently part of the circuit. The R3 pot slightly attenuates the denominator (DEN) input to produce a variable scale factor increase. Since the pot's effect can only increase the output, the series resistor R4 is included to attenuate the input slightly. With this in place, R3 can adjust the scale factor either upward or downward.
2) If the input is to be AC coupled, would R4 still be used?
Yes, the input coupling capacitor goes in series with R4. When you adjust the scale factor, use a test input signal of high enough frequency so that the coupling cap's reactance becomes negligible.
3) If using the post-filter, how to add AC coupling and scale adjustment?
Both these features can be added to the post-filter circuit of Figure 11, as shown below. Break the input lead at point A to add the AC coupling cap and R4. Break the denominator feedback path at point B to add the pot R3:
Best regards,
Bob