Write SQL query to solve given problem: 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?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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