I may be speaking out of place but the proper method is to use an op amp based adapter unit.
Generally companies like NOR Control or Autronica do the same.
It is basically a non inverting amplifier trimmed for response based on the output you desire.

The input 4 to 20 mA is given to one terminal I feed back adjusted to optimise the output.

Obviously there are critical valuse of the zero & span resistors but if you can wait a while & are interested then respond & I may even be able to get you a full circuit diagram.

Pls bear with me.