Eisenhower could run for a third term, but wouldn't run for a fourth given his health. Reagan successfully running any earlier than 1980 (with '76 as the most likely time) is actually a poisoned apple as he gets the blame for the economy and Iran rather than Jimmy Carter. Nixon is a distinct possibility if he wins in 1960, which is possible if information about one of JFK's affairs is leaked somehow or if JFK's health is worse. I could see him winning in '64, '68 and '72. As far as Clinton's concerned, he could beat Bush in 2000 despite the Lewinsky scandal and other issues, but 9/11 is likely to do him in IMHO. Long story short, Nixon (oh the irony) is the most likely candidate for a four term President post-FDR, assuming no shenanigans like Watergate.