The answer to the crossword for the term 'Neighbor of South Dakota' is Iowa, which is a four-letter word. Iowa is a state located in the Midwestern United States, bordered by the Mississippi River to the east and the Missouri River and Big Sioux River to the west. It is the only state whose east and west borders are formed entirely by rivers. Iowa is the neighbor of South Dakota to the east, with the two states sharing a border along the Big Sioux River.