So, you’re applying for a job and the recruiting agency asks you for your resume, wants to know when and you graduated High School and want to know your last 4 digits of your Social Security Number (for tracking purposes). You may not realize it, but you’ve just turned over enough information to allow someone to figure out your Social Security Number.
The first 3 digits of your SSN indicate the State the SSN is issued from and the next 2 numbers (the group number) are only issued in certain years. If you were born after 1988 a SSN was issued automatically at birth. Prior to that most people got a SSN when they got this first job (generally sometime between their 16th and 21st birthday). When you consider that the average person attends high school in the state they were born and graduate high school between 17 and 19 you can narrow it down so there's a 9% chance the attacker can guess your SSN in 1000 guesses. That might sound safe but it's a trivial task to build a computer program to run the possibilities.
The last 4 digits are the part of your SSN that’s unique and not predictable by any algorithm.
So when organizations other than your employer, a back or the government ask you for your last 4, be aware of what you might be giving up.