What version of Excel are you using?

Try this and we'll tweak it once we know what version of Excel you're using.

Let's assume the sequence of numbers is 1 to 6.

Enter this array formula** in B1:

=SMALL(IF(ISNA(MATCH(ROW(INDIRECT("1:6")),A$1:A$4,0)),ROW(INDIRECT("1:6"))),ROWS(B$1:B1))

** array formulas need to be entered using the key
combination of CTRL,SHIFT,ENTER (not just ENTER).
Hold down both the CTRL key and the SHIFT key
then hit ENTER.

Copy down until you start getting errors. We can remove the errors but need to know what version of Excel you're using so we can use the best method.