What a treat is was! The nuns were wonderfully welcoming, and were eager to show us around after the Mass. This is actually the "new" basilica. The first one was started in the 4th century, and was on the site of an elaborate villa. It was added to over the centuries, and was all but completely destroyed during the Norman Sack of Rome in 1084. A new, smaller basilica was rebuilt, using the original apse, and it was consecrated in 1116.
Who were the Four Crowned Martyrs? We don't know their names, although different traditions have assigned various names to them. According to The Passion of St. Sebastian, they were four unnamed soldiers, Christians all, who refused to offer pagan sacrifices and so were killed under the Emperor Diocletian. Their bodies are in the crypt, each in its own ancient sarcophagus.
Over the centuries there have been beautiful frescoes added, including some dating back to the 13th century. The basilica is on the Caelian Hill, and it's a marvellous place to visit.