A special commission sent by President Carter to El Salvadoran security forces to the murder there of four American women missionaries. The commission probably will recommend that Carter release $25 million in suspended U.S. aid to help moderate forces gain the upper hand in that strife torn Central American country reliable sources said yesterday.

However, the sources stressed, a final decision about resumption of the military and economic as assistance has not been made yet and will depend on U.S. policymakers being assured of two things: that Salvadoran authorities will make a genuine effort to solve the murders, and the Salvadoran government will be recognized in a way that puts the moderates in sufficient control to carry out far-reaching reforms and halt the excesses of rightist sympathizers within the military.

Specifically, the sources continued, this almost certainly would require reaching an agreement to dissolve the civilian-military junta that has ruled El Salvador for the past 15 months and replace it with an executive whose principal figures would be Jose Napoleon Duarte, civilian leader of the Christian Democrats, as president, and Col. Jaime A. Gutierrez, a moderate army officer, as commander of the armed forces.

The sources said this is the likely result of the mission dispatched by Carter to El Salvador last Saturday after the women -- three Roman Catholic nuns and a lay social worker -- were murdered Dec. 2. The commission was headed by William D. Rogers, a former under secretary of state for economic affairs and assistant secretary for inter-American affairs under former president Gerald R.Ford, and William G. Bowdler, the present assistant secretary for inter-American affairs.

Rogers returned here Tuesday and Bowdler yesterday and they began working with State Department and White House officials on their report. The sources said the mission could present its findings and recommendations to Carter -- who will make the final decision -- as early as today.

The sources said the aim of the mission had not been to fix responsibility for the killings, but to determine what Salvadoran authorities are doing about the investigation and to assess the country's internal situation.

On the basis of its findings, sources said, the mission is expected to tell Carter that no real evidence has been uncovered linking the Salvadoran national guard, a militia organization, or other Salvadoran security forces to the murders.

But, the sources continued, the mission also will report that there are several circumstantial matters relating to the discovery and handling of the bodies by the national guard, and delays in reporting the killings up the chain of command, that require further investigation and clarification.

The Salvadorans have appointed a commission consisting of a civilian minister and three officers, chosen for their untainted reputations, to carry out this investigation. In addition, the envoys will tell Carter, the Salvadoran authorities have agreed to permit U.S. representatives and an impartial outside body such as the Organization of American States' Human Rights Commission to monitor the investigations.

U.S. policy until the murders had been to support the junta in hopes it could reconcile the country's warring leftist and rightist factions. U.S. officials have conceded, however, that the junta has been unable to control rightist sympathizers in the military. Security forces also have abetted a campaign of wholesale terrorism and murder.

Despite that situation, the State Department is known to feel that cutting off U.S. aid would abort efforts to strengthen the moderates and pave the way for a coup -- possibly led by Col. Carlos Edugenio Vides Casanova, commander of the national guard -- that would put the country completely under rightwing control.