What were the four border states that did not join the confederacy?

Delaware, Maryland, Kentucky and Missouri. Maryland tried to join the Confederacy, but Lincoln had the governor and the legislators arrested and put in prison without trial, and suspended the writ of habeus corpus to get away with it, to keep them from seceding. The Maryland leaders were replaced with pro-Union men. After seeing what happened to Maryland, Delaware did not try. Maryland sent soldiers and units to both sides, though, and so did Kentucky and Missouri.