The SI unit of Stefan's constant is

From Stefan's law, the total energy emitted per second by a unit area of a black body is proportional to the fourth power of its absolute temperature, i.e.

$E \propto T^4 or E= \sigma T^4$

where $\sigma$ is the Stefan's constant. Thus 

$\sigma = \frac{E}{T^4}$

∴   SI unit of $\sigma$ = $\frac{SI unit of energy per second}{SI unit of area \times {(SI unit of temperature)}^4}$

                        $= \frac{watt}{{\left(meter\right)}^2 \times {\left(kelvin\right)}^4 } = W m^{-2} K^{-4}$

Hence the correct choice is 4