Pornographic film star John Holmes was found innocent yesterday of the murders of four people who were beaten to death last summer in Laurel Canyon, Calif. Holmes, 37, also was found innocent of attempted murder in the near-fatal beating of a woman who survived the July 1 attack.

Defense attorney Earl Hanson, who rested his case without calling one witness, told the eight-man, four-woman jury that Holmes should be found innocent because he was forced at gunpoint to lead the killers to their victims. Although an earlier court ruling stated that Holmes could not argue duress as a defense, Hanson told the jury that duress was a "significant" factor in its decision on whether the porno star had the intent to kill anyone.

New York Mayor Edward I. Koch was absent from City Hall yesterday because he spent the entire day appearing as a witness in the trial of a lawsuit brought by Dr. Michael M. Baden, the city's former chief medical examiner. Koch has been sued both personally and as the mayor of New York.

The nonjury trial is being conducted by U.S. District Judge Charles S. Haight in Manhattan's Federal Court. The suit, stemming from Koch's removal of Baden from his $50,000-a-year post in 1979, alleges that the mayor and others had "stigmatized his Baden's professional name and reputation" by publicly revealing the charges against him. Baden, now a deputy medical examiner on Long Island, is asking several million dollars in damages.

The board and director of the Institute of Museum Services met in closed session yesterday for the first time in the IMS' four-year history. The session, at which grant applications for fiscal year 1982 were considered, was closed by IMS director Lilla Tower, to the consternation of at least one board member.

Tower's authority to close the meeting was unclear. Before the meeting, board member George Seybolt said, "I have no understanding that the meeting is going to be closed . . . My understanding is that the only way the meeting could be closed is by a vote of the board or a committee of the board."

But Tower told the five members of the 15-member board who attended yesterday's meeting that it was neither a meeting of the IMS board nor of a board committee. Rather, Seybolt said she told the board members, it was "a meeting of people she's called together to advise her" on the grant applications under consideration.

Although the National Endowment for the Arts and the National Endowment for the Humanities regularly meet in closed session when considering grant applications, the IMS has not followed that practice, largely because board discussion of grant applications is really pro forma approval, according to Seybolt. He said Tower's explanation for closing yesterday's meeting was that she "felt the information under discussion was financial and confidential."

Nine of the 15 seats on the IMS board are empty through expiration of members' terms, and the Reagan administration has made no move to fill those seats. The vacancies leave the board without a quorum and therefore without the power to officially conduct business.

Several people who normally attend the IMS' open meetings were turned away from yesterday's meeting. These included Alexander Crary, aide for the arts to Sen. Claiborne Pell (D-R.I.); Irvin McManus of the National Endowment for the Arts, and Craig Black, outgoing president of the American Association of Museums.

Tower told a congressional committee earlier this year that she supports the administration's efforts to eliminate the agency she administers. The IMS, created during the Carter administration, makes grants to help museums pay operating costs. The Reagan administration budget for 1983 requested no funds for IMS.