Posers and Puzzles
19 Feb 03
Saying that they are spaced by two or four years immeditely gives it away. Although it should read "Separated by four or eight years." If you pose that question to others, you should lieave out that part of the hint.
Since the post above mine gives the answer, I may as well be more specific. The letters refer to the firsr letter of US presidents' forenames from Eisenhower forward.