Why did Franklin Delano Roosevelt have 4 different vice presidents?

Franklin Delano Roosevelt had four different vice presidents due to his lengthy presidency, which spanned four terms from 1933 to 1945. His first vice president, John Nance Garner, served from 1933 to 1941 but had a falling out with Roosevelt over policy disagreements. Henry A. Wallace took over in 1941 but was replaced by Harry S. Truman in 1945, as Roosevelt sought a running mate who could appeal to more conservative Democrats. Truman ultimately became vice president just months before Roosevelt's death in April 1945.