The term 'Rheinland Rejection' refers to the rejection of the Treaty of Versailles by the German state of Rheinland in 1919. The treaty was signed by the Allied Powers and Germany after World War I, and it imposed harsh restrictions on Germany, including reparations payments and limits on its military. Rheinland's rejection of the treaty was a major factor in the rise of Nazi Germany and the outbreak of World War II. The German word for 'NO' is 'nein, ' which is the four-letter solution to this crossword puzzle.