What is the last 4 digits of a Social Security Number?

Excuse me, could you please clarify something for me? I understand that Social Security Numbers are confidential and sensitive pieces of information, but I'm curious to know - if one were to have access to such a number, how would one determine the last four digits of it? Is there a specific pattern or formula that can be applied, or is it purely random? I'm asking purely out of academic curiosity, of course.