Doubt from electric flux

In illustration 5 what will be the flux through S3 surface it hasn't mentioned in their solution

it has mentioned flux through s1 and s4 is zero but what about s3 ?

There’s something absurd, both the axis are mentioned as x axis , S4 is written twice , S1 is calculated in the solution and then you are saying it is also given as 0


Although , for getting for eg S1 it will be = - surface opp to s1 as electric field is constant