Final answer:
Ambrose is the fourth 'Doctor' of the Church alongside Augustine, Jerome, and Gregory the Great. Augustine's influential works, including 'The City of God,' shaped Western theology, and although Boethius was significant, he wasn't designated as a Doctor of the Church.
Step-by-step explanation:
The fourth "Doctor" of the Church alongside Augustine, Jerome, and Gregory the Great is Ambrose. Ambrose, like the other Doctors, played a pivotal role in establishing early Christian theology and doctrine. Notable among these figures is Augustine of Hippo, a Roman African and a preeminent theologian and philosopher whose works like The City of God and Confessions deeply influenced Western Christianity and philosophy.
Another key figure was Boethius, whose work Consolation of Philosophy had a significant impact during the Middle Ages. However, he was not designated as a Doctor of the Church. The title is given to Saints who have been recognized by the Church for having made a significant and lasting contribution to theology or doctrine through their research, study, or writing.