The honor for Sister Alphonsa of the Immaculate Conception, the first Indian woman to become a saint, comes as Christians increasingly have been the object of attacks from Hindu mobs in eastern and southern India.
Benedict's predecessor, John Paul II, had beatified Alphonsa during a pilgrimage to India in 1986. Beatification is the last formal step before sainthood, the Church's highest honor for its faithful. Alphonsa, a nun from southern India, was 35 when she died in 1946.
The other new saints are: Gaetano Errico, a Neapolitan priest who founded a missionary order in the 19th century; Sister Maria Bernarda, born Verena Buetler in Switzerland in 1848, who worked as a nun in Ecuador and Colombia; and Narcisa de Jesus Martillo Moran, a 19th century laywoman from Ecuador who helped the sick and the poor.
"May their examples give us encouragement, their teachings give us direction and comfort," Benedict said in his homily.
When Benedict read the Latin words proclaiming all four saints, many in the crowd in the square waved flags from the homelands of the newly honored.
An aide held a white umbrella over the pope to shield him from a hot sun as he sat in a chair on the steps of St. Peter's Basilica to receive Mass gifts during the ceremony.
Many people associate Mother Teresa, a Nobel Peace Prize-winning nun, with India. The ethnic Albanian came to India as a young woman to work with India's most desperately poor. She died in 1997 and John Paul beatified her in 2003.