noun
four-year university/college
Abbreviation of 四年制大学. Used in educational contexts to refer to institutions offering four-year undergraduate programs.
彼は四大を卒業して、すぐに就職した。
He graduated from a four-year university and immediately got a job.
prefix
the big four; the four major
Prefix used to refer to the four most prominent or largest entities in a category, e.g., 四大文明 (the four great civilizations).
四大文明は、メソポタミア、エジプト、インダス、黄河の四つです。
The four great civilizations are Mesopotamia, Egypt, Indus, and the Yellow River.
The full, formal term for a four-year university; 四大 is its common abbreviation.
Abbreviation of 四年制大学 for the noun sense; prefix sense derives from 四 (four) + 大 (big/great).