As Teddy suggests. Data storage requiremnets are defined in the table, presentation requirements are defined in the presentation layer (forms & reports).
Access muddies this a bit as:-
1) you can specify the datatype - if your data is an integer why are you using a decimal data type?
2) you can specify a format and inout mask (which wuill be pulled through into forms and reports
3) you can apply formatting in queries using VBA functions such as round() fix() and Int()
I agree. You should save the data as a Long Integer and then display it as the five digit 'number'.
I would also suggest you use a function that creates the five digit number for every report, form, etc. That way if something changes you don't have to look in more than one place to make the change (when they need to add a sixth digit).