4-20ma inputs

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.