SQL ORDER BY with CASE with UNION ALL

Put everything in another SELECT:

SELECT * FROM (  SELECT "Field1" AS field_1, "Field2" AS field_2,  "Field3" AS field_3, "Field4" AS field_4  FROM "TableName"   WHERE condition  AND other_condition  UNION ALL  SELECT "Field1" AS field_1, "Field2" AS field_2,  "Field3" AS field_3, "Field4" AS field_4  FROM "TableName"   WHERE yet_another_condition  AND yet_another_other_condition) As AORDER BY CASE field_1    WHEN 'A' THEN 1    WHEN 'B' THEN 2    WHEN 'C' THEN 3    ELSE 4END

or, better, use the alias in ORDER BY, as it is passed at the end of the UNION:

  SELECT "Field1" AS field_1, "Field2" AS field_2,  "Field3" AS field_3, "Field4" AS field_4  FROM "TableName"   WHERE condition  AND other_condition  UNION ALL  SELECT "Field1" AS field_1, "Field2" AS field_2,  "Field3" AS field_3, "Field4" AS field_4  FROM "TableName"   WHERE yet_another_condition  AND yet_another_other_condition  ORDER BY CASE field_1    WHEN 'A' THEN 1    WHEN 'B' THEN 2    WHEN 'C' THEN 3    ELSE 4  END

The first one does not work because you should do

ORDER BY CASE field_1

The "Field1" is only available in a single subquery, and after you make the UNION with a common alias, you cannot refer to that column as "Field1" any more.