Are you saying that 4mA offset with a fixed resistor is different from 4mA generated from PWM.Both using the same Arduino VCC.Please explain.Leo..
Allenhurst described how to add calibration trimmers. But, I wouldn't do that on the emitter resistor -- it will heat up, slightly, which may skew the resistance, and thus, the output current. Also, thermal expansion and contraction could nudge the wiper on the trimmer, knocking it out of calibration.
If you decide to calibrate you don't need to buy more accurate resistors - the cal process deals with this.
Maybe. Even at 20mA, the resistor only dissipates 40mW total and the pot at 25 ohms setting 10mW - not much. But adjusting the input divider is also perfectly viable. There's a hundred ways to skin this cat.