Four persons -- the State Department's nuclear weapons policy chief, his wife and an unmarried couple -- were shot to death yesterday afternoon in two unrelated incidents in Fairfax County.

Two were the apparent victims of a murder-suicide; the others, a double slaying that occurred barely two miles away.

Fairfax police said the body of Marvin W. Humphreys, 44, director of the State Department's Office of Nuclear Polcy, was found late yesterday with that of his wife, Lynda, in their $200,000 brick rambler in the 6200 block of Lakeview Drive over-looking Lake Barcroft. Police said they believe that Humphreys, a career Foreign Service officer, shot his wife and then turned the gun on himself.

Less than a half hour earlier, police were summoned to a home in the 3700 block of Rose Lane where they found a young man and a woman dead of gunshot wounds. A third person who lived at the house told investigators he had discovered the bodies of his two friends, one of whom police tentatively identified as Billy Tumarkin, 25, shortly after 4:15 p.m.

Police said they had no suspects in the killings and had not found a murder weapon late yesterday. Investigators said they were withholding final word on the identities of the couple until relatives are notified.

All four of the victims were shot in the upper bodies, but officers said they had uncovered nothing that would indicate the killings were linked. Neither family knew each other, detectives said.

At the Humphreys home, police said they found the body of Mrs. Humphreys faced down in the kitchen, with her husband lying a few feet away in the living room. Both bodies were discovered by a coworker of Mrs. Humphreys, who failed to appear for work yesterday at a Northern Virginia real estate firm.

Colleagues of Humphreys at the State Department said he had been depressed recently by marital troubles. The eldest of the Humphreys' two sons, Steven, a 21-year-old senior at the Massachusetts Institute of Technology in Boston, said in a telephone interview last night that his parents had been planning to get a divorce.

"They seemed to be working it out amicably," he said. "They were planning to get a divorce, with a separation first. They seemed to have decided it was the best thing."

Steven, who celebrated his 21st birthday yesterday, said he had visited the family home over Labor Day weekend earlier this month and had spoken to his father by telephone last week.

"He seemed fine," he said. "I really wish I could rationalize it."

The younger Humphreys son, David is a junior at Yale University in New Haven, Conn.

A nuclear weapons policy expert, Humphreys had recently returned from a 3 1/2 week conference in Geneva that was convened to review the Nuclear Nonproliferation Treaty. He ranked as the third man in the U.S. delegation to the conference.

"It comes as such a shock -- he's always been the unflappable Marv," said one of Humphreys' supervisors in the department's Bureau of Politico-Military Affairs. "If he ws having problems, it didn't reflect on his work."

A coworker described the prematurely gray Humphreys as "one of the few people who really looked like he worked at the State Department," and said the career Foreign Service officer had shown no hint of depression on the job.

Another colleague said Humphreys did not show up at work yesterday and that his office began getting telephone calls late in the afternoon from neighbors who were troubled when police and ambulances arrived at the home. A neighbor who called the office said he was told Humphreys had called in to say that he would be late to work.

"He was a very self-disciplined person, very private," said a State Department colleague. "He didn't welcome intimacy. He seemed so self-controlled.

Lynda Humphreys, 42, worked at Fairmac Realty in Alexandria where she was regarded as one of the more successful sales people. George Overbey, her boss and the firm's principle broker, said last night that Marvin Humphreys wanted his wife to quit her job so that she could spend more time at home.

"He tried to get her to quit. She said, 'I'm cut out for this, and the kids need the money for school' . . . he couldn't take it," Overbey said.

Neighbors said the family has owned the home since their return from the American Embassy in Paris in 1975. Mrs. Humphreys was described as a quiet woman who served as chairman of the local beach committee responsible for maintaining the Lake Barcroft waterfront.

Humphreys was a 1958 graduate of Harverford College and has a master's degree from the Fletcher School of Law and Diplomacy. He joined the State Department in 1959, spoke French and Italian and served in Stuggart and Bonn as well as Parks. Before going to work in the nuclear policy and operations department, he was in the bureau's arms control and disarmament agency.

Police said that a special police team had been sent to the Rose Lane home where the other two bodies were found about a year ago to investigate a disturbance. Police said they could not immediately provide any details of that incident.

Neighbors said Tumarkin drove trucks and heavy construction equipment and had grown up in Annandale. His mother owns the house where the shooting ocurred but no longer lives there, the neighbors said.