As was mentioned by using a current signal the resistance of the line is not important, therefore conductors lengthes can be very long. Most 4-20mA devices have an acceptable voltage range of 15 to 35 volts DC.
The reason that they start with 4mA is for "loss of signal" monitoring. If the PLC sees 4mA is knows that it is at zero value, but if it sees 0mA it knows the signal has been lost and can generate an error.
