We added a validation function to the process that imports gifts to Batch in version 4.9 (released May 2021) and upon review have discovered that this validation is checking to see if the credit card number is in a valid format. When only the last four digits are mapped to this field and gifts are imported to a Batch, users will see the exception, "Invalid credit card number specified."
The developers will change the validation for this field in the next version, but unfortunately, we will not be able to release that change to the Blackbaud hosted environment until Q3 of this year.
In the meantime, you can either import directly (bypassing the batch) or to import that data to another field (such as the Gift Reference, a Gift Attribute, etc.).