Lawyers in the murder conspiracy trial of convicted arms dealer Edwin P. Wilson argued for four hours yesterday behind closed doors in Washington's federal court.

District Court Judge John H. Pratt barred the hearing to reporters and the public. Attorneys and the judge would not say what was being argued in the closed court session.

In two earlier trials, Wilson has argued unsuccessfully he was working with the CIA while selling arms and terrorist expertise to the government of Libya in the late 1970s, and has asked that classified information be introduced in court to support his contention.

The Classified Information Procedures Act requires that any classified information attorneys for either side may want to use must first be approved by a judge in a private session.

Jury selection is scheduled to start today in the trial here on charges that Wilson and Francis E. Terpil, his alleged partner in the Libyan arms deals, plotted in 1976 to hire assassins for $1 million to have a Libyan dissident living in Egypt killed. Terpil, also a former CIA employe, is a fugitive.

Prosecutors have charged that Wilson, while living in Virginia six years ago, attempted to recruit three Cuban exiles as hit men to kill the dissident, Omar Abdul Muhayshi. Wilson later paid $30,000 for expenses to one of the Cubans in the District, according to a grand jury that has indicted him on two conspiracy charges.

The slaying was never carried out.

Wilson, under heavy guard yesterday by marshals, faces up to 10 years in prison if convicted. He was sentenced earlier to 32 years imprisonment after convictions in Alexandria and Houston on arms and explosives smuggling charges.