Need help combining four columns into one for each record

Need to combine WO_DAYS_LEFT, SO_DAYS_LEFT, RO_DAYS_LEFT, RW_DAYS_LEFT columns into one column named DAYS_LEFT for corresponding record.
How would I do this?

SELECT        dbo.BA_VIEW_SHIPPING_ORDERS.SM_NUMBER, dbo.BA_VIEW_SHIPPING_ORDERS.STATUS AS SM_STATUS, dbo.BA_VIEW_SHIPPING_ORDERS.ORDER_TYPE, 
                         dbo.BA_VIEW_SHIPPING_ORDERS.ORDER_NUMBER, dbo.DEPARTMENT.DEPT_NAME, dbo.BA_VIEW_SHIPPING_ORDERS.PN, 
                         dbo.BA_VIEW_SHIPPING_ORDERS.DESCRIPTION, dbo.BA_VIEW_SHIPPING_ORDERS.SHIP_NAME AS CUSTOMER_NAME, 
                         dbo.BA_VIEW_SHIPPING_ORDERS.SHIP_VIA_CODE, CASE WHEN BA_VIEW_WO_SUMMARY.CONTRACT_TAT IS NULL 
                         THEN 21 ELSE BA_VIEW_WO_SUMMARY.CONTRACT_TAT END - dbo.BA_VIEW_WO_SUMMARY.NET_TAT AS WO_DAYS_LEFT, DATEDIFF(DAY, 
                         dbo.BA_VIEW_SHIPPING_ORDERS.DUE_DATE, GETDATE()) AS SO_DAYS_LEFT, DATEDIFF(DAY, GETDATE(), dbo.BA_VIEW_REPAIR_ORDERS.SM_ENTRY) 
                         AS RO_DAYS_LEFT, DATEDIFF(DAY, GETDATE(), dbo.BA_VIEW_REWORK_RO.SM_ENTRY) 
                         AS RW_DAYS_LEFT
FROM            dbo.BA_VIEW_SHIPPING_ORDERS LEFT OUTER JOIN
                         dbo.BA_VIEW_REWORK_RO ON dbo.BA_VIEW_SHIPPING_ORDERS.ORDER_NUMBER = dbo.BA_VIEW_REWORK_RO.RO LEFT OUTER JOIN
                         dbo.BA_VIEW_REPAIR_ORDERS ON dbo.BA_VIEW_SHIPPING_ORDERS.ORDER_NUMBER = dbo.BA_VIEW_REPAIR_ORDERS.RO LEFT OUTER JOIN
                         dbo.BA_VIEW_WO_SUMMARY ON dbo.BA_VIEW_SHIPPING_ORDERS.ORDER_NUMBER = dbo.BA_VIEW_WO_SUMMARY.WO_NUMBER LEFT OUTER JOIN
                         dbo.DEPARTMENT ON dbo.BA_VIEW_SHIPPING_ORDERS.DPT_AUTO_KEY = dbo.DEPARTMENT.DPT_AUTO_KEY
WHERE        (dbo.BA_VIEW_SHIPPING_ORDERS.SM_NUMBER LIKE 'SHA%') AND (dbo.BA_VIEW_SHIPPING_ORDERS.STATUS NOT IN ('SHIPPED', 'CANCELLED'))

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