Unfortunately, the photo does not show the stems and leaves of the plant.  Nonetheless, I think it is certainly a mustard (belonging to the Cruciferae or Brassicaceae) based on the four petals and overall architecture of the flowers and inflorescence.  It is most likely Lunaria annua, which is commonly called "Money Plant" or "Honesty."  If it were flowering later in the season, it could be Hesperis matronalis, commonly called "Dame's Rocket."  If it is Lunaria, the pods will be round, and the leaves will roughly heart-shaped with toothed edges.  If Hesperis, the pods will be long and thin and the leaves will be elongated and lance shaped.

I hope this helps.