This is one 4 lb. antique cast iron window sash weight. 

Antique window sash weights are used to hang double hung windows in old houses. A window sash is suspended from ropes (sash cord) tied to sash weights. To properly hang a double hung window, the upper sash should be hung with weights slightly heavier than the actual weight of the sash, and the lower sash should be hung with weights slightly less than the actual weight of the sash. This is to ensure that the upper sash stays up, and the lower sash stays down. With window sash less than 12 lbs., the weight variation from counter weights to window sash weight can be a half pound. For window sash weighing over 12 lbs., the variation should be a pound difference. That means a 15 lb. upper window sash should be hung with two 8 lb. sash weights and a lower 15 lb. sash should be hung with two 7 lb. sash weights. 

For more information about restoring old windows we recommend The Window Sash Bible by Steve Jordan.