Three or four hooded attackers entered the friary and bound and gagged the friars, the oldest of whom was 86.
One managed to raise the alarm two hours later, when he regained consciousness after being beaten.
"They unleashed an incredible level of violence against them," Gabriele Trivellin, in charge of Franciscan friars in the area, told the Corriere della Sera newspaper.
"It was wild and gratuitous violence because they did not resist the attack at all," he said.
The youngest friar injured - 49-year-old Sergio Baldin - is currently in hospital in a coma after suffering severe head injuries. The other three, who range in age from 76 to 86, are expected to be released from hospital in a month.
Police believe the motive may have been robbery, as some cash was stolen from the friary during the attack.
But the only object of great value in the building - a golden crown which decorates a statue of the Madonna - was placed behind protective glass after the original was stolen two years ago.