At about 17:25 EST, a Boeing 737 operating as Canadian Airlines Flight 528 experienced a near midair collision while maneuvering to land at La Guardia Airport. The Boeing 737 was on downwind for runway 4, at 4,000 feet, when the crew caught a glimpse of three lights. The first officer saw the other airplane and had enough time to realize they were not going to collide, but not enough time to execute an evasive maneuver. In addition, the airplanes TCAS went from TRAFFIC to MONITOR VERTICAL SPEED to CLEAR OF CONFLICT in approximately 2 seconds. The other airplane reportedly was a Cessna 172, IFR at 5,000 feet. The pilot of that airplane apparently saw the 737 at his 12 oclock position and perceived it to be at the same altitude. To avoid a collision, the Cessna pilot executed a descent, and at 4,000 feet MSL, the two airplanes passed 0 feet vertically and 500 feet horizontally from one another.