Depends of how the 4-20mA sensors are powered.
Theoretically, you can just use a 250 ohm resistor, and measure the voltage drop across it, assuming no common mode voltage issues.
The problem is that most power supplies these days do not use a transformer, and unless the power supplies to the sensors and the Pi's are all isolated, you could run into some real problems due to common mode voltage issues that could easily fry the Pi and ADC's.
4-20mA sensors are industrial sensors, so unless you are very knowledgable, your best bet is to:
a) forget about using a Pi, and use PLC's with 4-20mA analog input modules fitted
b) hire someone familiar with interfacing to 4-20mA signals, common mode voltage issues, isolation etc
This is NOT a project for someone new to interfacing.
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