Who was the only president to serve 4 years?

The following U. S. Presidents served only one complete four-year term:

1. John Adams (1797-1801)
2. John Quincy Adams (1825-1829)
3. Martin Van Buren (1837-1841)
4. James K. Polk (1845-1849)
5. Franklin Pierce (1853-1857)
6. James Buchanan (1857-1861)
7. Rutherford B. Hayes (1877-1881)
8. Benjamin Harrison (1889-1893)
9. William H. Taft (1909-1913)
10. Herbert Hoover (1929-1933)
11. Jimmy Carter (1977-1981)
12. George H. W. Bush (1989-1993)