FOUR New York City police officers accused of murdering an unarmed Guinean immigrant in a hail of gunfire last February were acquitted of all charges on Friday. Officers Sean Carroll, Edward McMellon, Kenneth Boss and Richard Murphy were cleared of second-degree murder and other charges in the slaying of Amadou Diallo, a street vendor from Guinea. The plainclothes officers claimed they fired 41 bullets at the victim on Feb. 4, 1999, after he refused orders to halt for questioning and pulled out a black object they thought was a handgun. Diallo was hit 19 times in the vestibule of his Bronx apartment before the defendants realized the object was a wallet.