Question

Why 4ma-20ma ?

Answer 1

I suspect the question is in relation to measuring devices and transmission within and control system. Basically you can't use zero because you can't detect a malfunction transmission. The standard industry is 4 - 20mA and 1-4 V. A system using these ranges in transmission of temperature, pressure and other system measurements which are relied to control panels and controllers.

Answer 2

Ideally a output of an electric/electronic circuit can be simulated with a series Thevenin voltage/impedance or it's corollary a parallel Norton/impeadance. The impedances are the same and
IN = VTH/impedance. Personally if one is valid so should the other, though I remember some saying the contrary.