2 typical 4...20mA current source diagrams enclosed.
One with a rail-rail opamp, one 'old fashioned' discrete version.
Edit
Or ( it's not quite clear from your question) if you want to do things the other way around - ie control the 4-20mA, use smoothed PWM from an arduino to drive the 3rd circuit.
/edit
Allan
20mAd.pdf (18.1 KB)
20masrc.pdf (17.6 KB)
20ma.pdf (16.1 KB)