I recently read in the Nicene and Post-Nicene Fathers series regarding St. Ambrose, that he was one of the "four Latin doctors of the Church". I've never heard that phrase. I could probably Google it, but I would rather ask here.
Adam is correct. And the four Greek Doctors of the Ancient Church are two of the Cappadocians (Basil and Gregory of Nazianzus), the great preacher Chrysostom and the man who stood contra mundum, Athanasius. There are a few dozen "doctors," reckoned in this way, altogether.
The word "doctor" is just a form of the Latin (doceo) for "teach," so the doctor is "one who teaches." These then were reckoned especially to be the church's teachers. The office of doctor in the church is the office of teacher, an office that I hold as a minister of Word and sacrament.