Chilean opposition politicians pushing for a return to democracy in that country fear their cause has suffered a setback during the recent tour of six Latin American countries by Jeane Kirkpatrick, the U.S. ambassador to the United Nations and the Reagan administration's prime exponent of "quiet diplomacy" on human rights issues.

Two days after Kirkpatrick wound up her visit there, publicly praising the policies of Chile's military president, Gen. Augusto Pinochet, Chilean security forces Tuesday summarily expelled four prominent opposition politicians from the country.

The four represented a cross section of moderates among leaders of the centrist and leftist opposition parties, which have reportedly moved closer in recent months to concerted opposition against Pinochet, who took power in a military coup eight years ago and will rule until 1989 under provisions of a new constitution.

One of the political leaders, former justice minister Jaime Castillo Velasco, president of the Chilean Commission on Human Rights, had tried unsuccessfully to arrange a meeting with Kirkpatrick to introduce her to representatives of an organization of relatives of the "disappeared" -- 600 presumed political prisoners who disappeared after arrest by the Chilean secret police.

"The visit of Ambassador Kirkpatrick was received by the government as meaning the enthusiastic and unconditional support of the Reagan administration for the Chilean government and as the explicit derogation of the United States' human rights policy," charged Claudio Orrego, a Christian Democratic former member of the Chilean parliament.

Orrego maintained in a telephone interview from Santiago that Kirkpatrick's visit was a "tragedy for the democratic opposition in Chile . . . Ambassador Kirkpatrick comes here with the policy of quiet diplomacy and the next day the government commits one of the worst violations of human rights of the last few years."

The Chilean government, in announcing the expulsions, said the four opposition leaders had ignored "repeated warnings" and "continued to maintain a defiant attitude which the government cannot tolerate."

The statement accused the four of supporting a "Marxist" labor union movement, a reference to a petition they signed Monday with 27 other Chileans protesting the imprisonment of two officers of an outlawed opposition labor organization.

Since the 1973 coup, Chile has been one of the principal Latin American targets of human rights criticism. Relations with the United States became especially rocky after Chilean secret police agents were indicted by a U.S. grand jury on charges of plotting the 1976 assassination in Washington of exiled opposition leader Orlando Letelier.

In a press conference in Santiago after interviews with Pinochet and government leaders, Kirkpatrick was quoted in news service reports as saying the United States intended to "normalize completely its relations with Chile in order to work together in a pleasant way," and declined comment when asked about Chile's human rights record.

She said the Letelier case "is solved and what remains to be dealt with is no impediment whatsoever" to improving relations with the United States. The administration already has moved to lift economic sanctions against Chile imposed under the Carter administration.

Charles Lichenstein, alternate U.S. ambassador to the United Nations, said he could not speak for Kirkpatrick's reaction to the expulsions of the four dissidents, who were helicoptered to Argentina. "Insofar as it will embarrass her and President Reagan and the whole policy of quiet diplomacy, obviously someone made a miscalculation. It's very difficult to speculate about motives," he said. "You can argue that the explusions were just an unhappy coincidence."

Kirkpatrick, who was scheduled to return to the United States late yesterday, also visited Venezuela, Uruguay, Argentina, Ecuador and Peru.