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They are the Four Papal Basilicas in Rome:
Top-left: St Peter's Basilica:
Top-right: Basilica of Saint Paul Outside the Walls - most photos - eg. the one on Wikipedia - show the more detailed Western face, but the drawing above depicts the Northern face:
Bottom-left: Archbasilica of St. John Lateran:
Bottom-right: Basilica of Saint Mary Major:
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The specific names have already been mentioned, but those are the four "major" basilicas of Rome.
Often devoted pilgrims will make it a point to visit all 4 on a trip to Rome. (This is especially true during a "holy year" in which each of those basilicas has a special door that is only open during those events.)