The former president of a California-based explosives firm, who admitted that he conspired with two former CIA employes to supply weapons and training to a school for terrorists in Libya, was sentenced yesterday in U.S. District Court to serve four months in prison.

Assistant U.S. Attorney E. Lawrence Barcella said in court yesterday that a federal grand jury is currently invesitgating information that the defendant, Herome S. Brower of Pomona, Calif., also helped ship 40,000 pounds of highly powerful plastic explosives to Libya in the fall of 1977.

Barcella told Judge H. Pratt that it is believed to be the largest nonmilitary shipment of plastic explosives out of the United States now known to federal officials.

Brower, 61, allegedly purchased the plastic explosives and assisted in arranging for shipment to Libya, Barcella told Pratt. Federal law prohibits the shipment of such explosives on commercial or cargo aircraft, as well as exporting it without required permits and selling it to specified countries, including Libya.

Law enforcement officials learned specifics about that shipment after a grand jury last April indicted Brower and the two former CIA employes, Edwin P. Wilson and Frank E. Terpil, on charges that the Libyan government hired them to run a secret terrorist training school there.

Brower pleaded guilty last December to conspiracy to violate various federal arms transporation laws. In addition to the four-month prison term, Pratt placed Brower on three years probation and ordered him to pay a $5,000 fine. Brower has been cooperating with federal officials investigating the case.

Wilson, 51, who lived with his family on a huge farm in Upperville, Va., is a fugitive believed to be living in a villa in Tripoli, Libya. The United States does not have an extradition treaty with Libya, making that country a safe haven for Wilson while he faces criminal charges in the United States.

Terpil, who lived in McLean, had pleaded not guilty to the charges following indictment but then failed to show up for his trial in New York last September on unrelated charges that Terpil and a codefendant tried to sell 10,000 machine guns to police undercover agents there. Prosecutors believe that Terpil left the United States and is now traveling in the Middle East.

The government has alleged that Brower's role was to help Wilson and Terpil meet the terms of a contract they had with the Libyan government in 1976. In its indictment last April, the grand jury said that in addition to supplying weapons and training, Terpil and Wilson also recruited an American citizen in 1976 to assassinate a Libyan ex-patroit who a year earlier had been involved in an unsuccessful plot to overthrow the government of Libyan leader Col. Muammar Qaddafi. The assassination attempt described by the grand jury was never carried out