A written plea for help from four Americans held hostage in Lebanon yesterday relieved their families' fears that they had been shot to death but prompted the Reagan administration to restate its policy of not "negotiating" with terrorists even to save lives.

Some relatives, shaken by reports Thursday that the hostages had been killed by firing squad, echoed the captives' appeal to the president to make a deal for their release.

"As they say in the letter, the captors themselves want a peaceful resolution," said Peggy Say of Batavia, N.Y., sister of hostage Terry A. Anderson, chief Mideast correspondent for the Associated Press. Anderson sent a personal message to her and other relatives yesterday.

"The administration has made promises to the families which seem empty in view of the fact that the hostages are not home," she said. "If they mean what they say, they will begin to talk to the captors."

At least one relative of a hostage disagreed. "It sounds callous, but I agree with the administration. What the hostages are writing is not under their own volition," said John Jenco of Joliet, Ill., brother of the Rev. Lawrence Jenco, a Catholic priest taken hostage Jan. 8.

"I'm sure that even with my brother, his feeling would be not to give in," John Jenco said. As for the reports that the hostages had been killed, Jenco attributed them to terrorists "trying to get the families all riled up to pressure the government . . . . At least now we know they're alive."

White House spokesman Larry Speakes said at a press briefing yesterday that "the president's policy has not changed as far as negotiating with the terrorists, and will not change."

But he added, "We're making a restatement of policy rather than trying to respond to a letter which we don't know is authentic."

Officials were concerned that their restatement of policy not jeopardize the hostages, one said. They noted that "talks" with the captors have always been a possibility.

"The United States does not get involved in negotiations but is ready, of course, to talk about the release of the hostages . . . ," State Department spokesman Bernard Kalb said. "We remain in touch with a number of parties in the region on this whole issue."

The central difference between negotiating and talking, he said, is that "with negotiations there may be somehow an implication of concession or a deal . . . . That is out."

The United States might not be able to make a deal if it wanted to, since the key to freeing the Americans is in the hands of another government. The Islamic Jihad, which claims to be holding the Americans, has demanded that President Reagan force Kuwait to release 17 terrorists jailed for the bombings of U.S., French, and Kuwaiti installations in late 1983.

The open appeal addressed to the president was part of a package of letters delivered yesterday to the Associated Press bureau in Beirut and signed by four of the six Americans kidnaped in Lebanon in the last 23 months and still held there.

AP staff members in Beirut said they recognized the handwriting of Anderson, who was kidnaped March 16. And Jean Sutherland, wife of hostage Thomas Sutherland, dean of agriculture at the American University of Beirut who was kidnaped June 9, verified his.

Although the signatures of two hostages -- William Buckley and Peter Kilburn -- were missing from yesterday's letters, officials said they "are working on the assumption" that all six are alive.

According to his kidnapers, Buckley, a U.S. diplomat abducted in March 1984, has been killed, although U.S. officials say they have no evidence of that. And Kilburn, an American University librarian who disappeared last December, may be being held by a separate group, officials say.

"We remind the kidnapers that we hold them fully responsible for the well-being of their captives," Speakes said yesterday.

In his letter to his family, Anderson wrote: "However distasteful it might be, Reagan must negotiate if he cares at all about our well-being. And he must do so soon. William Buckley is dead after 1 1/2 years in captivity. I don't want to share that fate."

The State Department called relatives of the hostages early yesterday with news of the letters.

Their arrival "definitely relieves the fears that we were all experiencing" on Thursday, said Eric Jacobsen, son of hostage David Jacobsen, director of the American University Hospital in Beirut who was kidnaped May 28.