What is the recommended isolation IC for isolating a 4-20 ma current signal from a 3.3 v u-processor
Normally, a 4-20 mA sensor is not isolated. To connect a 4-20 mA sensor to an MCU, I would suggest just using a resistor. A 125 Ohm resistor will give 2.5V full scale. If the sensor can source current, connect the resistor to ground.
To isolate a 4-20 mA sensor, I would suggest using the Si8900 Isolated ADC with a 125 Ohm resistor. The Si8900 needs an isolated 3.3V power source.
If the MCU is the sensor, then this is a different problem. If the MCU has a DAC, the DAC can drive a trans-conductance amplifier. If the MCU does not have a DAC, use a PWM to generate a voltage, then a trans-conductance amplifier. If the MCU is powered by the current loop, the MCU should be connected such that its current consumption is included in the 4 mA base load.
If isolation is required, I would suggest using a PWM with a digital isolator, and then a trans-conductance amplifier on the isolated side. I would suggest using one of our Si87xx opto-coupler replacement isolators.