FOUR ARE DEAD and more than 30 injured in Thursday's bombing at the Conservative Party conference in Britain. Prime Minister Margaret Thatcher, the apparent target of the blast, was uninjured but at least one of her Cabinet ministers was badly wounded. Responsibility for the murders has been claimed by the Irish Republican Army, but that is nothing new. The terrorist group has had a hand in many of the 2,500 deaths in Northern Ireland, including those of scores of innocent women and children. It has committed at least one political assassination inside a church and claims to have murdered prominent British figures, including Lord Mountbatten and conservative MP Airey Neave.

Some Americans share heavily in the responsibility for these killings, and it is time they were made to face up to it. Those who raise funds for the IRA in the United States can no longer hide behind the fiction that their contributions go to the families of political prisoners in Ulster. Irish Prime Minister Garret FitzGerald has deplored the fact that "There are still people in the United States who are failing to comprehend the situation in (Ireland) and are willing to give aid for the purpose of sending arms and ammunition to murder people . . . " Members of Congress, especially those of Irish ancestry themselves, have urged constituents not to contribute to the cause for the money goes for guns not food and clothing, for violence and not charity.

Two weeks ago, the Irish navy intercepted a trawler in territorial waters off the coast of Cork. It was loaded with seven tons of arms and ammunition bound for the IRA. Security sources here and in Ireland say the arms left a port on the east coast of the United States in a freighter that was tracked by an American satellite, making the capture possible. The cargo included rockets, handguns, submachine guns, automatic rifles and a large assortment of ammunition. Ireland's Minister of Justice Michael Noonan says that American sympathizers of the IRA had paid for this huge consignment of weapons. American money is used, he says, to "kill people north and south of the Irish border."

We may never know whether the money that bought explosives for the Brighton massacre was raised in Boston or the Bronx, but the link between fundraising here and violence there is apparent. The peace and security of two nations, long and close friends of the United States, are at stake, and British and Irish lives are threatened. Americans of conscience must reject this violence and reject association with the killers whose cowardly acts of murder and mayhem are despised on both sides of the Atlantic.