How can I read a 4-20ma sensor, I can find no references to it
Arduino can't read a current directly, only a voltage in the 0 to 5V range.
So you must convert your variable current into a variable voltage.
Google "Current to voltage conversion"
Look at this: http://elchem.kaist.ac.kr/vt/chem-ed/electron/circuits/itov.htm
But be sure to check if the voltage that comes out of it given your current is in the 0 - 5V range before connecting it to an Arduino analog input.