The accident occurred at the weekend near the coastal town of Palinuro, south of Naples in a cave known as the Blood Grotto.
The divers have been named as Briton Douglas Rizzo, 41, who lived in Rome; Italians Andrea Pedroni, 40 and Susy Covaccini, 36; and Greek Panaiotis Telios, 23.
Divers enter the cave through a tunnel at a depth of about 13m and exit through another tunnel higher up. However, a third tunnel leading off the cave ends in a sealed chamber.
A group of eight or nine divers entered as planned but visibility was lost when sediment was kicked up.
While some managed to find the correct tunnel out, the four who lost their lives went down the one leading to the closed chamber.
"It's an easy cave but there is a dangerous tunnel that people never use,” local diving school owner Roberto Navarra told Il Messaggero newspaper. “Four people swam into that channel."
Survivor Marco Sebastiani, who runs his own diving school in Rome, told Il Messaggero that “agitation of the least experienced took hold” after the sediment was disturbed.
"We couldn't find the exit,” he said. “At a certain point I managed to find my way. I took as many people as I could with me and we swam towards the light, which grew bigger all the time.
“When I came up, I looked around to count us and I realised that Susy, Andrea, Douglas and Panos weren't there."
An attempt was made to dive back down to locate the lost divers, but the lack of visibility made searching impossible until some time later, when sediment had settled.
A police investigation has been ordered by an Italian magistrate.
The Blood Grotto is so named for its walls tinged red by bacterial growth. It is one of more than 30 underwater caves visited regularly by divers in the Cilento and Vallo di Diano National Park.
Founded in 1991, the park is a UNESCO World Heritage site.