The relatives of four Americans being held hostage in Beirut left meetings in Washington with Reagan Administration and Syrian officials with renewed optimism that negotiations will continue for the captives' release. The families met for one hour in the White House. Present were Eric Jacobsen of Huntington Beach, the son of hostage David P. Jacobsen of the American University in Beirut; the families of Father Lawrence Jenco, a Jesuit priest, and Thomas Sutherland, dean of agriculture at American University. Two other American hostages in Beirut are William Buckley, an official of the U.S. Embassy, and Peter Kilburn, librarian at the American University in Beirut. Buckley has been reported killed, although the reports have not been confirmed.