 Uniform-charge estimate at a capacitor corner

Four uniform square charge-sheet pairs, rotated around a common corner, contribute equal normal fields there. Their superposition is a large central plate pair, so each contributes one quarter of its field. This gives $|E_n|\simeq \sigma /(4{ϵ}_0)\simeq V/(4d)$ when the bulk charge-voltage relation is used. It is an approximation for prescribed uniform sheets, not an exact equipotential-conductor result. Reflection in the mid-plane does give zero tangential field for either model.