Relatives of four of six Americans still held captive in the Middle East said yesterday that the Rev. Benjamin Weir gave them "reassuring" news about their loved ones but expressed belief that his release was a message that the hostages might be executed if their captors' demands are not met.

Weir told them, they said, that he had seen four of the hostages last Saturday in Beirut just before his release, and he brought hastily written letters from each.

Weir said, however, that he had not seen Peter Kilburn, a librarian at the American University in Beirut, and William Buckley, a political officer at the U.S. Embassy there. The other hostages are the Rev. Lawrence Martin Jenco, Terry A. Anderson, David P. Jacobsen and Thomas M. Sutherland.

The family members, who are to meet here today with Vice President Bush, said they were particularly comforted when Weir told them that he and those four were allowed to be with each other and worship together.

Jenco's sister, Sue Franceschini of Joliet, Ill., said it was "very reassuring" to hear that he and Weir had each other's company. Jenco was head of Catholic Relief Services in Beirut.

"I think we got a sense of the companionship that all of them felt with each other," said Peggy Say of Batavia, N.Y., sister of Anderson, chief Middle East correspondent for Associated Press. "I'm just very pleased to know that they do have each other, that they're in reasonable health and they're not being brutalized."

Kilburn's niece, Patty Little of Aptos, Calif., said she remains optimistic that her uncle is alive, although he has serious medical problems. Kilburn, 60, suffered a stroke two years ago that left him partially paralyzed and has a heart condition and high blood pressure that require monitoring and medication.

She said she is putting "everything on hold now" until the others are released. "I think if they get his medication to him he can survive the boredom," she said.

Buckley, 57, a bachelor from Medford, Mass., apparently has no close family ties in the United States. No relatives have tried to contact either government officials or relatives of the other hostages.

At a news conference yesterday, Weir said he and the four hostages he met were allowed to worship together while blindfolded and eventually were allowed to take off the blindfolds and talk with each other in locked rooms.

Say said she had "this picture in my mind of what [Weir] described as Terry holding the Bible up . . . and reading to David Jacobsen. It gives me a good warm feeling."

Many of the other relatives sat together during the news conference and, when Weir mentioned the death threats, several clung to and comforted each other.

Jenco's brother, John, said he is "cautiously optimistic." Asked if he supports the U.S. government's approach, he said he has not been as critical as some of the other relatives, who said they do not think that the Reagan administration has been aggressive enough in seeking the captives' freedom.

"I'd say let's give in" to the captors' demands, Jenco added.

He said he believes that the Americans are "too valuable to harm. If nothing is done [to meet the demands], they might kidnap more Americans, but I don't think they will kill" the ones now held.

Some relatives said Weir's release has given them a "kind of a resurgence of energy," in Say's words.

She and Franceschini said they take seriously the captors' threats to execute the hostages if Kuwait does not release 17 captive Shiite Moslems.

"I have had a feeling for some time that they may sacrifice one of them to show us that they mean what they said," Say said. "If they see that we're doing nothing, and that means the government and the hostage families, I don't think that would be very wise."

Say said the relatives "want to work with Vice President Bush. We have never wanted to be enemies of the administration or critical of them. We have a common goal here."