The amplitude of electric field in a parallel light beam of intensity $4W{m}^{-2}$ is

Intensity ,   $I=\frac{1}{2}c{ϵ}_o{E}^2$   where $c$ and $E$ are the speed of light and amplitude of electric field respectively.

$4=\frac{1}{2}\times 3\times 10^8\times 8.85\times 10^{-12}\times |E{|}^2$

$⟹|E|=55.5N{C}^{-1}$