Chapter 22 - Gauss's Law - Problems - Exercises - Page 747: 22.27

The electric field for $S_2$ and $S_3$ is zero while for $S_4$ is $E = \sigma / \epsilon_{o}$

Both $S_2$ and $S_3$ are outside the plates, so they enclose zero charges while $S_4$ is between the two plates, so the electric field for $S_4$ is \begin{gather} E A=q / \epsilon_{o}\\ E = q / \epsilon_{o} A\\ \boxed{E = \sigma / \epsilon_{o}} \end{gather}