format the cell as text and it will work.
go to format>cells>text
I just tried it using:
Format Cells -> Number -> Special -> Zip Code
And it worked fine. Try using the built-in function under Special (it has Zip Code and Zip Code plus 4) instead of doing you own custom format. Hope this helps
What did you use with custom format? If you select format 00000, 1 will appear as 00001. The other alternative is use =TEXT(4976,"00000"), then copy and paste values. It will then be stored as text and not a value.
HTH
Richard
put ian apostrophe in fron of the number e.g '0497