The fourth line takes the same ridgepole and lifts it. Legge's translation describes the beam curving upward, with good fortune, and attaches a condition: if the subject looks elsewhere for support, there will be cause for regret. A beam that arches upward is one being carried from beneath rather than merely holding out against its load, and the whole difference between this line and the sagging third is where the pressure is coming from.
The stroke is strong in an even place, incorrect by the ordinary rule, and it opens the upper trigram Dui. Against the third line's flawless correctness and bad outcome, this pairing of an incorrect position with good fortune is deliberate. What matters in Great Preponderance is not whether a line sits where the rule prefers but whether it is answered, and this line is answered by the yielding first stroke at the base of the figure, the same line that laid mats of mao grass beneath what had to be set down.
The condition is the sharper half of the line. Regret follows not from weakness but from casting about for additional support while the one real support is already in place. A structure carried at a single sound bearing is stable; the same structure propped at several points that do not agree with each other is not. The text is unusually specific about this, and the specificity is the counsel. The arrangement works, and it works only while it is left alone.