If Zip Codes are in Numeric you will find that in the states of CT and MA, the left most zeros are truncated.
A Zip Code field should be Text, so that the leading zeros will remain.
You can construct an Update Query in Access to fix these.
First make a backup field called ZipFix so that you have the Zip field in its original form to go back to, just in case. Make this a Text field
Use an Update Query to fill the ZipFix field with the same contents of Zip. It will convert the digits to text.
Use an Update Query to process those Zip codes that have 1 digit, then 2, then 3, then 4.
First click on the DataSheet icon to see which records will change.
Then click on the ! Run button to process the changes