In case you want to sum DomesticOrder, Forecast, ExportOrder and IntercoOrder for same product in same month, try:
Code:
SELECT ProductCode, SUM(Month1) AS [Month1], SUM(Month2) AS [Month2], ... FROM
(SELECT dom.ProductCode, "Domestic Order" AS [OrderType], dom.Month1, dom.Month2, ... FROM DomesticOrder_Crosstab dom
UNION
SELECT fc.ProductCode, "Forecast" AS [OrderType], fc.Month1, fc.Month2, ... FROM Forecast_Crosstab fc
UNION
SELECT exp.ProductCode, "Export Order" AS [OrderType], exp.Month1, exp.Month2, ... FROM ExportOrder_Crosstab exp
UNION
SELECT int.ProductCode, "Interco Order" AS [OrderType], int.Month1, int.Month2, ... FROM IntercoOrder_Crosstab int)
GROUP BY ProductCode
Rem. In case you have in some MonthN columns Null values, use Nz(MonthN,0) in SUM() parts.