Authorities believe all four victims were homeless.
Police said they were searching for a man they suspect could be behind all of the attacks. They were reviewing subway security camera footage as part of the investigation. All four stabbings occurred along the A subway line.
One of the victims was discovered dead on a train in Queens at about midnight on Friday with several stab wounds to his neck and torso. Two hours later, a 44-year-old woman was found stabbed to death in the subway in upper Manhattan.
The two nonfatal attacks — one involving a 67-year-old man and the other involving a 43-year-old man — also occurred in upper Manhattan.
In response, NYPD officials said they would deploy an additional 500 officers into the subway system throughout the city as a safety measure.