The robber, who earned his nickname by wearing a distinctive cloth over his face, is believed to have hit banks in Bellevue, Woodinville, Shoreline and Bothell.
Three of the four robberies were in October, with the latest being the Widbey Bank in Bothell on October 23, according to the FBI.
Authorities believe the robber is a white man who stands between 5'9" and 6-feet tall and has a thin build. During the robberies, he covers his face and wears the hood of his black or orange sweatshirt over his head. He also wears clear, latex gloves.
Anyone with information about the robber is asked to call Crime Stoppers at 1-800-222-TIPS (8477).