DOYLESTOWN, Pa. — A marijuana dealer gave police a grisly account of killing four men on his family's farm, saying he crushed one of them with a backhoe after shooting him and tried to set three of the bodies on fire in a metal bin with the help of his cousin, according to court papers filed Friday.Cosmo DiNardo, who graduated from a Catholic prep school two years ago, said he killed a former schoolmate when he arrived with $800 to buy $8,000 worth of pot. DiNardo, who's charged along with his cousin, said he shot another man in the back as he tried to run away.DiNardo, 20, pinned one of the deaths on his cousin, who was charged Friday, although the cousin told police that DiNardo shot all four of the victims.The only motive disclosed by investigators was that DiNardo said he wanted to set the victims up when they went to the farm to buy marijuana. One man vanished July 5, and the others vanished two days later.Three of the slain men were buried at the farm, in Solebury, in an oil tank that had been converted into a cooker.