The four American presidents referred to in the crossword solution are John Adams, John Quincy Adams, John Tyler, and John Kennedy. John Adams was the second president of the United States and served from 1797 to 1801. He was a Founding Father and a leader of the American Revolution. John Quincy Adams was the sixth president of the United States and served from 1825 to 1829. He was the son of John Adams and was a diplomat, lawyer, and statesman. John Tyler was the tenth president of the United States and served from 1841 to 1845. He was a member of the Whig Party and was the first vice president to assume the presidency upon the death of a sitting president. John Kennedy was the 35th president of the United States and served from 1961 to 1963. He was a member of the Democratic Party and is remembered for his leadership during the Cuban Missile Crisis.