If, as I assume, the 738 has the same system in this regard as the 732, the reason for the IV opening on selecting either engine bleed air switch OFF is to avoid assymetric wing de-icing. It's true that this will give a single source to both packs, which is undesirable because now neither pack will work efficiently on the reduced flow, but the QRH will correct this. However, you'll still have the sufficient air from the one engine that's now supplying it to maintain pressurisation; it doesn't matter whether that air has passed through two packs or one.