I am trying to identify the buildings in this picture.
The one on the top left is in Vatican City, and the rest are also Catholic related but I'm not sure where they are.
All the pictures are from a postcard.
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:
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.)