The same as in the last four digits of my social security number, which some banks use as a "password" for phone banking!
When I went to college your SSN was your student id number. Yikes! Some professors wised up to that and when posting test scores they would only do the last four digits of your SSN.
However that ignores that the first three digits are easily guessed as its based on the state you
received your number in. Since it was a state school you could surmise that the person was one of 5 numbers. Then all you had to do was guess the two digit middle number.