Which four presidents died of old age?

Nobody ever dies of old age. Except in Science Fiction, there is no age by which everyone must die (although it seems nearly impossible to exceed the age of 120 at this point).

Of the 38 U.S. Presidents who have died to date, four of them were in their 90's at the time. Gerald Ford and Ronald Reagan died at the age of 93, and John Adams and Herbert Hoover died at the age of 90.