linking fields which may be incomplete

I have a table of 3,000 disciplinary cases from the past four years which my
school has tracked. The identifier we used was a. student's name, and b. the
last four digits of their SSN.

I am now need to find their full SSN. We have a "warehouse" of master
tables, which I can link to. Many of these tables contain the full SSNs of
the students. The question is, is there anyway to link the fields together
(one field including only the last four digits of the SSN, one field which is
the whole SSN) in a way that I can find the full SSN of only the students in
my table of 3,000 disciplinary cases?

Any help would be much appreciated.
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