Ok. Now be sure and test this. :) (I don't have an iSeries box, so I can't)
DELETE FROM LIBA/mytable F1
WHERE RRN(F1) > (select MIN(RRN(F2)) FROM mytable F2 WHERE F1.col1=F2.col1 AND F1.col2=F2.col2 AND F1.col3=F2.col3 AND F1.col4=F2.col4 AND F1.col4='2005-01-01')
That should be pretty close...
Kent