New York police were investigating whether a cab driver found shot to death in his taxi was the fourth victim of a serial killer. Rafael Montes De Oca, 38, was found in his car at 11:55 p.m. Saturday in the Bronx, police said. He was shot in the head at close range, as were three other private car-service drivers killed in the Bronx since March 7. Police had not ruled out that the killer believed responsible for the previous three slayings may have struck again, said police spokesman Hugh Barry. But officers investigating the latest killing were leaning against connecting it to the others. "There are certain consistencies in the other cases which are not present in this one," said Lt. James Malvey.