The entire fifth floor of the J.E.R. Carpenter-designed prewar co-op building at 4 East 66th Street (also known as 845 Fifth Avenue) has hit the market for a whopping $48 million, or $6,400 per square foot. The 15-room apartment is owned by hedge fund founder Daniel Nir and his wife, Jill Braufman. The couple bought the place from hotelier Robert H. Burns in 2007, paying $29 million. Burns had originally listed it for $37 million. It features a private elevator landing, two wood-burning fireplaces, and, for the time being at least, a ping-pong table in the living room.