,SUM(CASE WHEN AcTr.AcNo <= '3999' THEN AcTr.AcAm*-1 ELSE 0 END)
FROM F0001.dbo.AcTr AcTr, F0001.dbo.Actor Actor
WHERE (Actor.CustNo = AcTr.R4) AND (Actor.CreDt>='20140101') AND (AcTr.AcYr='2013') AND (AcTr.AcNo>='3000' AND AcTr.AcNo<='8999')
GROUP BY AcTr.R4
What is the relation between AcTr and Actor ? the join field ?
Can you post some sample data ? for testing ...