I know you said all the columns are NVARCHAR(4), but check again - you can use the following code to check:
SELECT DATA_TYPE FROM INFORMATION_SCHEMA.[COLUMNS]
WHERE TABLE_NAME = 'thirdtablename' AND COLUMN_NAME = 'Yearcolumname'
If it returns NCHAR, that is the issue; the data type needs to be NVARCHAR(4), not NCHAR(4)