WESTWEGO, La.—A 4-year-old boy in Louisiana, sitting in the back of a car with two younger siblings, fatally shot himself in the forehead with a gun he found while his mother and another adult smoked marijuana in the front seat, authorities said. An autopsy Monday found that Jarion Walker’s gunshot wound was self-inflicted, Jefferson Parish Sheriff Joseph Lopinto said at a news conference late Monday, The Times-Picayune/The New Orleans Advocate reported. “It was a contact wound to the forehead, right above the right eyebrow,” Jefferson Parish Coroner Gerry Cvitanovich said. The shooting happened about 10:45 p.m. Saturday in the city of Westwego, a suburb of New Orleans. Jarion was sitting in the back seat with two siblings age 22 months and about 1, authorities said. The adults, who admitted to smoking marijuana at the time, said they heard the shot, Lopinto said. The little boy was rushed to a hospital, …