The Fourth Coast is a reference to the Great Lakes region, with a particular reference to Michigan, since it has "four" coastlines, and is the only state that does. A few local businesses in Kalamazoo have adopted the name (such as the Fourth Coast Cafe). It seems that the term is gaining in popularity and is mentioned often in blogs when referring to Michigan.
The term Fourth Coast is also applied to the Saint Lawrence River in North America. Some business in those regions have adopted the name (such as Fourth Coast Entertainment, located in Canton, New York).
References
Fourth Coast Wikipedia(Text) CC BY-SA