After more than a year of secretive, often hesitant investigation into the 1980 slaying of four American churchwomen, Salvadoran authorities suddenly are preparing to release extensive information on the case and begin legal proceedings against the alleged killers, according to sources close to the proceedings.

Salvadoran officials declined to confirm publicly reports from sources here and in Washington that one or more of the six Salvadoran National Guardsmen alleged to have participated in the crime have admitted involvement. The reported statements from the accused followed the arrival here in the past two weeks of an FBI polygraph expert requested by the Salvadoran government.

News of a break in the case comes as the Reagan administration was compelled today to certify to Congress that efforts are being made to move in the case against the killers and that the general human rights situation here is improving.

The Dec. 2, 1980, slaying of three nuns and a lay worker is perhaps the single most emotional factor in the disquiet felt by many people about the U.S.-backed government here.

Although the government soldiers were arrested last April, there were repeated charges that the armed forces, who jointly rule El Salvador with civilians, purposely were holding up the investigation. In May, U.S. investigators working with the Salvadorans were able to link two of the suspects to physical evidence in the case--a single fingerprint and a single shell casing. Salvadoran officials argued that, under local law, this was not enough for a conviction and by August, the investigation was at a virtual standstill.

When Salvadoran President Jose Napoleon Duarte and other government officials visited the United States in September, however, they were confronted with American officials, congressmen and a general public that convinced them that the investigation had better resume its momentum and start showing results or U.S. assistance to the junta would become politically impossible to continue, officials close to Duarte said.

Because of allegations that government soldiers had been responsible for the killings, there were indications that "there was ongoing tension between members of this government who just wanted to let the case die and those who wanted to continue pushing," one official close to the case said. "And on top of that, the people on the investigating commission really have no experience with this kind of thing."

Yesterday, Defense Minister Jose Guillermo Garcia announced that a special three-member commission looking into the case finally has finished its work. Garcia also reportedly told one journalist that the accused will be presented to the courts soon.

During the month preceding today's administration certification, leftist guerrillas fighting against the government here were engaged in their own efforts to prove that the junta could not pass the test.

To prevent a cutoff of U.S. military assistance to El Salvador, the Reagan administration was obliged to show that El Salvador is making a concerted effort to improve human rights, is "achieving substantial control" over its armed forces, making progress with economic reforms and looking for a political solution to the civil war.

Earlier this month, rebels took reporters from major American newspapers, including The Washington Post, to the scene of a massacre allegedly committed by government troops in December. Bodies apparently have been left unburied for more than a month and were readily visible to the correspondents, and witnesses were made available to them for accounts of the action.

There were other attempts to influence the certification as the Union of Salvadoran Peasants, often at odds with the junta--especially since the repeated, uninvestigated killings of many of its leaders--issued a scathing attack on crucial aspects of the government's bogged-down land-reform program.

The American Civil Liberties Union, meanwhile, presented the White House this week with a detailed and highly critical report on the general human rights situation in El Salvador.

On the military front, guerrillas claimed today to have wiped out a major portion of the Salvadoran Air Force in an attack Wednesday. The insurgents' underground radio station described the action as a commando operation completely undetected by American-designed security at the Ilopango air base outside the capital until after their bombs started going off. At least 10 helicopters and airplanes were damaged badly.

In Mexico, the five members of the guerrillas' Farabundo Marti National Liberation Front high command renewed their offer of a negotiated end to the war.

In a four-page letter delivered to the White House yesterday, the guerrillas used restrained language to describe the junta's alleged repressive acts, including the recently reported massacre, and the closing off of any peaceful route toward change. The guerrillas insist that the war here is a domestic problem that should not be part of Washington's confrontation with Moscow, a reference to the Reagan administration's frequent accusations that Cuba and members of the Soviet Bloc are supplying the rebels here with arms.

"The above considerations lead us to suggest respectfully to you the need for a change in your policy toward El Salvador," the letter concluded. "We only demand the right that you help us to resolve our problems without intervention."

The junta here, with backing for its position from the Reagan administration, has rejected negotiations in favor of elections as the only political solution to this country's devastating civil strife.

As if to underline the dangers of the electoral process here, a leading member of the conservative National Conciliation Party was assassinated by unidentified gunmen Tuesday night.