4-20 mA output from Arduino for VFD
It will only work practically if the supply ground is isolated and an op-amp is used that is "single supply" such as an LM358. You could use the LM741 if you added a negative supply, such as -5V but that would be a lot of trouble to use an obsolete part. Using a more negative supply than -6V or so would expose Q2 to damage if the inverse Vbe rating is not high enough. 

So you feed 5V from the PWM for 20mA (100% PWM) and 20% PWM (1V) for 4mA.

Maximum load resistance is about (12V-5.2V)/0.02A = 340 ohms to allow a full 20mA out.