If your question is really about Outlook, of course you should post it to an
Outlook discussion group.
On the off-chance that you simply mistyped and you are really asking about
Excel, you really have not given enough details to answer the question.
Interpreting you literally, the answer could be as simple as:
formatted with the Custom format 0000 .
Note that that might generate some 4-digit numbers with leading zeros. You
might consider those to be 1-, 2- or 3-digit numbers.
Also note that, unfortunately, the number will change every time you edit
the workbook -- any cell(!). You might find that undesirable.