Add Flex Fields to Duplicate check feature

Summary:

We have created EFF for Candidate Job Application to collect only "Last four Digits of SSN" and added it to the extra Information block on career site. Now , is there any possibility to use the EFF and perform Duplicate check and filter out the candidates who has entered same information in the EFF's

If adding EFF is not possible to Duplicate check feature, Can we use the standard National Identifier field to collect only Last Four Digits of SSN instead of full SSN in the Job Application flow?

please let us know if anyone has worked on a similar requirement.