Why is zero represented by 4mA in 4 - 20 mA industrial control systems?
In a 4-20mA current loop (which appears to be what you are talking about), the minimum 4mA current is set not for any measurement reasons per se, but to provide a guaranteed operating current for the electronics at the far end of the loop. This allows them to operate with no additional power supply at the far end, saving the extra wiring that would be needed. Often the transmitter will be a pressure sensor, or optical gate, or thermometer. 

The 4mA is a compromise between low power consumption for the system, and enough power for the sensor to operate. There is no more magic behind the exact figure of 4mA than (say) 240v for mains voltage. It is a reasonable value, which over the course of time has been found useful, so has been supported by many different players, and become a standard.