In a small, cinderblock laboratory in Hawaii, Army scientists are carefully studying the remains of four American servicemen who they believe were killed in Vietnam.

The bodies, known by such designations as "X-10" and "X-15," are among the last unanswered questions of the Vietnam War. The scientists are trying to figure out who these men are, but they also are seeking an "unknown soldier"--an unidentifiable serviceman who could qualify as a symbol of America's war dead for burial at Arlington National Cemetery.

Army Secretary John O. Marsh Jr. is scheduled to visit the Hawaii laboratory today to check on whether his scientists are making any significant progress in identifying the four bodies.

It has been eight years since Congress directed the Army to reserve a crypt at Arlington for an unknown serviceman killed in Southeast Asia. That crypt sits unmarked alongside the tombs of unknown soldiers from World War I, World War II and the Korean War. A 2 1/2-ton white marble cover designed for the Vietnam soldier has been waiting in storage since the fall of Saigon in 1975.

At one time, the Army had recovered the bodies of 140 unidentified servicemen from Southeast Asia. Now, after years of piecing together their identities like parts of a giant puzzle, only four remain unidentified.

The scientists are looking for clues by matching the remains against the medical records of 2,433 servicemen who served in Vietnam and whose bodies have never been recovered. The process is arduous and fraught with difficulties.

The search for a new unknown soldier of the Vietnam conflict, which is being pushed by some veterans groups and members of Congress, may rekindle some of the conflicting emotions that swirled around the unpopular war. Some families believe that selecting an unknown Vietnam war soldier would be tantamount to abandoning the search for those missing in action who have never returned from Southeast Asia.

More than 70 House members, however, led by Rep. Bill Goodling (R-Pa.), have written to Marsh to ask that an unknown soldier be chosen as soon as possible. The Veterans of Foreign Wars also has thrown its weight behind the idea.

Army officials say they still are trying to decide whether any of the bodies in Hawaii meets the legal criteria for an unknown soldier: that the serviceman be an American, that he have been killed in combat in Southeast Asia and that his identity be unknown.

In addition, Assistant Army Secretary William R. Gianelli said the service has a nonbinding rule that at least 80 percent of the serviceman's remains must have been recovered.

"We only have four candidates at the present time out of all the people who were killed in Vietnam," Gianelli said. And in all four cases, he said, the Army has less than 80 percent of the remains of each body.

Gianelli said that while the 80 percent rule may have made sense in earlier wars, when there were great numbers of unknown soliders from which to choose, it may have to be abandoned as impractical. While many of the casualties from previous wars were recovered largely intact, the more deadly explosive weapons used in Vietnam left the Army with only partial remains of many bodies.

Many of those killed in World War II could not be identified because they had been buried in Europe for years, he said, while the Vietnam casualties were flown home and identified almost immediately.

There were 1,648 unknown servicemen considered for burial at Arlington after World War I and 8,526 such casualties after World War II, Army officials said. That figure dropped to 848 after the war in Korea.

Today's more advanced technology also makes the search for an unknown soldier more difficult. Army scientists can use more sophisticated techniques to identify only partial remains, and they are more reluctant to abandon the effort at any given time.

Col. Michael Vargosko, an Army spokesman, said Army personnel have spent the equivalent of 33 years on the case. As a practical matter, Gianelli said, the work will have to end at some point.

"There is increasing public and congressional interest in an unknown soldier from the Vietnam era," Secretary Marsh said in a recent letter to Rep. G.V. (Sonny) Montgomery (D-Miss.), chairman of the House Armed Services Committee. Marsh said he has "directed that the matter be restudied thoroughly."

The Army brought three of the bodies to Hawaii from Thailand in 1976. The fourth was returned by the Laotian government in 1978, but officials are less certain that this soldier was killed in combat.

One forensic expert familiar with Army procedures said that scientists can make a positive identification by matching the bone structure, skull, teeth, fingerprints or hair with medical and dental records. For example, he said they could determine age, sex and blood type by studying the bone structure. He said they also could make an identification by superimposing a photograph of the remains on earlier X-rays of a serviceman's skull or rib cage.

Rep. Goodling, a prime supporter of the memorial, wrote the Army at the urging of Michael Wherley, a 30-year-old Vietnam veteran from York, Pa. When Wherley took his wife and two children to Arlington in July 1980, he was dismayed to find that no serviceman from Vietnam was interred alongside the other unknown soldiers. Instead, there was only a small bronze plaque in the Memorial Amphitheater to commemorate those who served in Vietnam.

"It's another kick in the rear for Vietnam veterans, as if for some reason we don't deserve the honor," said Wherley. "We've gone through enough personal agony. The country should stand behind those who went and fought. People go to the cemetery and there's nothing to even explain why there's no Vietnam 'unknown.' "

Wherley, who runs a moving company, served in Vietnam in 1972 as a sergeant with the Army Special Forces. He said his most frightening moments came during a firefight at Bien Hoa, when he hid behind a large drum of gasoline. He survived without injury, but discovered later a good friend had been blown up by an antitank mine.

Army officials say the final decision on the unknown soldier, which is not related to the Vietnam Memorial now being planned for Constitution Gardens, will be made by Defense Secretary Caspar W. Weinberger.