SELECT '04-jan-2014' COL1, 'C' COL2, .002 COL3, null COL4 FROM DUAL

WITH t AS
(
SELECT '01-jan-2014' COL1, 'B' COL2, .001 COL3, .0006 COL4 FROM DUAL UNION ALL
SELECT '02-jan-2014' COL1, 'B' COL2, .002 COL3, .0007 COL4 FROM DUAL UNION ALL
SELECT '03-jan-2014' COL1, 'B' COL2, .003 COL3, .0008 COL4 FROM DUAL UNION ALL
SELECT '03-jan-2014' COL1, 'B' COL2, .002 COL3, .0009 COL4 FROM DUAL UNION ALL
SELECT '03-jan-2014' COL1, 'C' COL2, .004 COL3, null COL4 FROM DUAL UNION ALL
SELECT '04-jan-2014' COL1, 'B' COL2, .005 COL3, .0009 COL4 FROM DUAL UNION ALL
SELECT '04-jan-2014' COL1, 'C' COL2, .002 COL3, null COL4 FROM DUAL
)
SELECT  col1,
        col2,
        col3,
        case
          when max(case col2 when 'C' then col3 end) over(partition by col1) is null then col4
          when col2 = 'B' then col4 * max(case col2 when 'C' then col3 end) over(partition by col1)
          else col4
        end col4
  FROM  t
/

COL1        C       COL3       COL4
----------- - ---------- ----------
01-jan-2014 B       .001      .0006
02-jan-2014 B       .002      .0007
03-jan-2014 B       .003   .0000032
03-jan-2014 B       .002   .0000036
03-jan-2014 C       .004
04-jan-2014 B       .005   .0000018
04-jan-2014 C       .002

7 rows selected.