How do you use an Arduino to control a 4 - 20 mA transmitter?
I can recommend the Click boards from MikroElektronica (I am the chief engineer porting support for them, and the PIC32-based Clicker development board series, to the chipKIT™ system).

The 4-20mA-T Click board is controlled by the Microchip MCP4921 DAC. This is a common DAC family used in many Arduino projects and there are plenty of libraries around for them (mine is here though it's aimed at the 4822 not the 4921).

Basically you set the DAC output voltage which sets the current output from an XTR116 current loop transmitter. 

The XTR116 takes an input of 0 - 4.095v, and provides a convenient 4.095v reference voltage to the MCP4921.


Datasheet for the XTR116
Datasheet for the MCP4921
Manual for the 4-20mA T Click


The whole thing is isolated from your main board through an ADu M1411 to keep everything safe from industrial process noise.