A Number type field includes a comma in a 4-digit value in Vault reports.
This is working as designed. Number type fields display a comma if the value contains more than 3 digits.
The solution is to use a Text type field to reflect certain types of numbers, such as a calendar year. Users should not enter alphabetic characters into this field so that there are no issues with how this field is displayed in Vault.
Vault Field Documentation: About Document Field Administration