When I need to do this I use modules from Pepperl & Fuchs which are expensive and for use in hazardous areas.
If "intrinsically safe" is involved STAY AWAY from making ANYTHING on the safe side. Pepperl and Fuchs is a very good source for CERTIFIED intrinsically safe equipment.

If the reason for isolation is purely electrical the simplest way is a voltage to frequency converter followed by an optoisolator.

Generally they will have a 250 ohm resistor, one end to 24vdc the other to the 4-20mA loop device and the differential inputs measure across the resistor. The other end of the 4-20mA loop device connects to 0V.
not correct
The other end of one of the 4-20mA loop devices connects to 0V.

The 4-20 standard specify that several devices can be connected in series in the 4-20 loop.

Yes, I have been 'burnt' by the above and relying on any 4-20 device having a ground reference is incorrect, risky and will give you trouble.

On the subject of intrinsically safe, I once made a solution dosage control where the valves, controls and panel is intrinsically safe. The process control box which is not 'safe' is mounted outside the building. I really enjoyed once having to maintain it in a wind chill of -10F (~ -20C)

Erik