Who was president for only four years?

John Adams, John Quincy Adams, Martin Van Buren, John Tyler, James Polk, Millard Fillmore, Franklin Pierce, James Buchanan, Abraham Lincoln, Andrew Johnson, Rutherford Hayes, Chester Arthur, Benjamin Harrison, William McKinley, William Taft, Herbert Hoover, Gerald Ford, Jimmy Carter, and George Bush all served only one term (four years). William Henry Harrison, Zachary Taylor, James Garfield, Warren Harding, and John F. Kennedy all served under four years due to death by disease, poor health, or assassination.