Controlling a 0-20mA transmitter from an isolated PWM signal
Add a CMOS logic gate after the optocoupler/R2. That will buffer the output and the voltage will be more accurate and the time constant will be the same for on vs. off. 
split R4+R3 approximately evenly (11K-ish) and put the capacitor to IRET in the middle. You're not really filtering the current input well by putting the capacitor where it is. The time constant will be the parallel combination of the two times the capacitance, so about 5500 ohms * 1uF or about 5 or 6ms. Adjust capacitance as required. I'm guessing 47uF or 100uF might be a better choice for a mechanical valve actuator. You  probably want the ripple to be less than the hysteresis in the actuator control loop. 
Consider replacing the optocoupler with a logic-output type (eg. 6N137) which will be much faster (so a more true representation of the PWM on the isolated side), but retain the above two suggestions (unless the optocoupler has a CMOS output- the cheaper older ones tend to be bipolar). 
If you use the logic output optoisolator, you might want to consider upping the PWM frequency to a few kHz to reduce the size of the filter capacitor so a ceramic part can be used (and it will respond a bit faster).