The most recent acts of violence in El Salvador, which included the murders of four American Catholic women, have pushed U.S. Roman Catholics into a variety of protest activities believed to be unmatched by few -- if any -- reactions to past issues, including the controversy over abortion.

An angry new banner that appeared in historic San Francisco cathedral last weekend tells why. It reads: "U.S. DOLLARS KILL U.S. NUNS."

Prelates and priests all over the country in the last few days offered memorial masses for the three nuns and one lay woman found murdered in the Central American nation last Thursday. Then they shucked off their vestments and fired off letters and telegrams, demanding that the Carter administration cut off all aid to El Salvador, and that the incoming Reagan administration make a public commitment to strong and continuing support for human rights as a basic element of U.S. foreign policy.

Less than a month after he was elected president of the U.S. Catholic hierarchy Archbishop John R. Roach of St. Paul-Minneapolis sent telegrams to both the outgoing and the incoming administrations and released a public statement to the press. Then he sent wires to each of his fellow bishops throughout the country, urging them to follow his example.

According to the Rev. Simon Smith, the Washington-based Jesuit missions spokesman, recent events in El Salvador produced for the U.S. State Department "a flood of letters second only to Iran."

A State Department spokesman agreed that there had been "heavy mail" throughout the year bearing on El Salvador, with "a lot of it form letters, though a lot of it has been individual letters, too, particularly from religious orders." He added that because of the "lead time" involved, the volume of mail probably would increase greatly in the coming weeks.

Religious orders have indeed been active in generating protests. Maryknoll Sister Annette Mulrey, who heads the social justice office of the religious order to which two of the murdered nuns belonged, reported that as soon as the media reported that the four women were missing, "we were besieged with calls from all over the country from people asking, 'What can we do?'"

Mulrey said no formal response had been received from either Ronald Reagan or the White House, but that "we got back from the network [of diocesan social justice offices] that they just deluged Washington." She believes this outpouring from church representatives in this country is "one of the reasons the State Department sent the team of investigators" to El Salvador.

Smith said he had "some reservations over why we are making such a big thing over the death of four people, when as many as 9,000 have died there" since Gen. Carlos Humberto Romero was overthrown more than a year ago. "But," he said, "this is a teachable moment, so let's use it."

Actually, before the murder last March of Archbishop Oscar Romero (no relation to the deposed general), American Catholics as well as Protestant groups -- such as the National Council of Churches -- have been sharply critical of American aid to the regime that has been unable to control the escalating violence.

Early last month, Bishop Thomas C. Kelly, general secretary of the National Conference of Catholic Bishops in this country, echoed the appeal of Salvadoran church leaders in a detailed statement calling for an end to the $5.7 million in U.S. military aid.

In his homily last Saturday at a mass for the slain women, Washington Archbishop James A. Hickey charged that "the present Salvadoran government seems to have given some of those extremists a kind of license to kill. I have heard accounts, seen pictures of their victims -- shot through the head, often mutilated . . . With others from around the world I experienced the sacrilege of the murder of Archbishop Romero, and the final desecration of his funeral! As Christians and as Americans, we cannot stand by in silence while this senseless and deliberate violence continues."