Electric flux is expressed as ϕ=E⋅A=EAcosθ, here, A is area of the surface and θ is the angle between the electric field lines and the normal to A.
The flux will be only from DEFG surface as on the surface OABC field is zero and for rest of the surface, area vector is perpendicular to field.

So, ϕ=EA
=4000×(0.2)2×(0.2)2
=4000×16×10−4Vm
=640Vcm
