Ronald Reagan first fell below 40-percent approval early in 1983, but by August of that year, he had rebounded slightly to 43 percent. He continued rebounding through his reelection campaign and all the way until 1987, when he dipped as low as 43 percent during the Iran-Contra Affair.
At this point in his presidency, Jimmy Carter's approval rating was in the low 30s, having rebounded from the upper 20s earlier in the summer. Carter first dipped below 40 percent in early 1979. Americans briefly rallied around Carter later that year, when Iranians captured 52 American hostages in Tehran, but Carter's approval ratings returned to the 30s less than a year later, and he lost re-election.
Gerald Ford took office in August of 1974 after the resignation of Richard Nixon; his approval rating sank below 40 percent five months later.
For his part, Nixon was quite popular for most of his presidency, until Watergate. His approval rating fell below 40 percent in the summer of 1973.
Lyndon Johnson's approval rating sank below 40 percent for the first time at roughly this point of his first full term in the White House: August of 1967. By this time, the U.S. seemed trapped in an increasingly-unpopular morass in Vietnam. When he announced he would not run for re-election in March 1968, Johnson's approval ratings were in the mid-30s.