There's a list of the 4 papal these churches, which in addition to St. Peter's include St. John Lateran
[DOC] [TLE] St. Peter's Basilica - Vatican City - Sacred Destinations [PAR] St. Peter's tomb is under the main altar and many other popes are buried in the ... 
outside the square and down the street: look for Paul and John on the south side 
(on ...