I believe the author was writing of post WWII.
He said the incumbent was almost always reelected, and he’s quite simply full of it, including from WWII on. FDR’s fourth term was Truman’s first partial term, and he was eligible to run for a second time in his own right, but turned it down. That’s the guy’s first eight years. While in office, Truman busted a couple of nationwide strikes, in one case at least using the US Army to do it. During the first Berlin Crisis (the other two happened during the Bay of Pigs and the Missile Crisis) he told the USSR and the world, “We are going to stay, Period.” During his campaign against Dewey he did a whistle-stop tour, and at each stop he had a shill in the audience (someone on the campaign staff) who would whip up crowd enthusiasm at a rehearsed time — he’d yell, “Give ‘em Hell, Harry!” He spent his remaining years carping about every Republican politician who’d ever lived.
Eisenhower served eight years, and ran as a Republican because it was a better fit for him, but also because he was concerned about the two-party system. That’s one.
JFK-LBJ, eight years, no incumbent POTUS.
Nixon-Ford, eight years; Nixon won reelection, but didn’t serve his entire second term.
Carter, four years.
Reagan, eight years, that’s two.
GHW Bush, four years.
Clinton, eight years, that’s three.
GW Bush, new century, but eight years, that’s four.
Obama’s five.
Truman, Eisenhower, JFK, LBJ, Nixon, Ford, Carter, Reagan, Bush, Clinton, Bush, Obama, four out of twelve.