Among the vendors with an average credit rating, what is the overall total due amount of purchases made by the company to the vendor that isn't preferrerd if another vendor is available?
average credit rating refers to CreditRating = 4;  vendor that isn't preferrerd if another vendor is available refers to PreferredVendorStatus = 0; SUM(TotalDue);
SELECT SUM(T2.TotalDue) FROM Vendor AS T1 INNER JOIN PurchaseOrderHeader AS T2 ON T1.BusinessEntityID = T2.VendorID WHERE T1.CreditRating = 4 AND T1.PreferredVendorStatus = 0