WP All Import 4.0.3 is the 3rd maintenance release since 4.0 that fixes bugs present in 4.0.
You don't need to upgrade to 4.0.3 unless you are experiencing a bug that is fixed in the list below.
Here are the changes from 4.0 to 4.0.3, including the changes in 4.0.1 and 4.0.2:
* enhanced uploading of large files
* enhanced and optimized auto-detection of the root element
* enhanced taxonomies hierarchy settings
* enhanced file type auto-detection
* modified "existing Custom Field values" dropdown to only show a maximum of 10 values
* fixed significant bug that stopped the database schema from being upgraded from the 3.x structure to the 4.x structure on some web hosts, resulting in error messages when executing imports after upgrading from 3.4.x to 4.0 on certain web hosts
* fixed bug where wrong number of records would be displayed after changing an import source file
* fixed bug where the maximum log storage setting didn't apply to the database, resulting in too many logs being stored in the database
* fixed bug related to cron execution when "do not create new records" option is enabled
* fixed bug related to unlinking attachment sources when posts are updated/deleted
* fixed bug related to special chars in taxonomies/categories mapping