Assuming you require a 4-20 ma analog output to drive/control the air regulator, you are going to require a DAC (digital to analog converter). I have seen such a shield, 4 channels I think, in the past but can't think where it might be posted.
Yes, there is definitely a lack of analog interface solutions for Arduino systems where we want to control industrial devices.
I have it mind to develop some boards to do more with analogs, specifically analog inputs from industrial devices.
There are many circuits available on the web, but as retrolefty says, you will need a digital to analog convertor.
You can, again find many sorts of these with various interfaces, I2C, SPI and so on.
You will need a suitable driving opamp to then provide the 4-20mA signal.
Remember, that to drive the 4-20mA signal, you will want to supply the opamp with something other than your 5 volt Vcc supply. Commonly 24Vdc is used and the final stage of the 4-20mA driver modulates the current.