I assume that the field is a text field. Perhaps it contains something like employee numbers or SaleIDs.
A good method to fix the problem would be to contatenate more zeros to the data, and then use the right() function.
Lets say that you have "01", "001", and "0001" in a field. the expression x:right("00" & fieldname,4) would make all of these values into "0001".
I tested this approach and I must admit that it did not work perfectly. However, I have used it in other languages, and I think that some variation of it would work in Access sql.