Where the third line attached badly, the fourth attaches well and does so in the open. Legge's translation shows the subject seeking union with the party beyond himself, with good fortune attending firm correctness, and the small commentary identifies that party without ambiguity: the line follows the ruler above it. There is no concealment in the gesture and no hedging. The attachment is directed upward, past the crowded lower half of the figure, to the single firm line that gives the hexagram its center.
Correctly placed as a yielding line in an even position, the fourth opens the upper trigram Kan and stands immediately beneath the fifth. That nearness is what makes open allegiance the right move rather than a risky one. A line at this height that hesitated, or that looked sideways for allies among its equals, would be read as building a faction; this line instead commits directly to the center and is credited with good fortune for it.
The contrast with the second line is deliberate and the two are usually read together. The second attaches from within, at a distance, through correspondence. The fourth attaches from outside itself, at close range, through proximity and choice. Holding Together needs both. A union that works only in private conviction never becomes a public order, and the fourth line is where personal loyalty becomes visible enough to be counted on.