The fourth line carries the third line's contest forward, but changes its outcome. Legge's rendering shows the subject mounting the city wall, positioned to attack, yet unable to carry the assault through, and the text adds that returning to what is right brings good fortune. Where the previous line hid its weapons and watched from a distance, this line has advanced as far as open confrontation, only to find the attack itself impossible to complete, and the text treats that failure not as defeat but as an opportunity to abandon a contest that should never have been pursued.
This line opens the upper trigram Qian, the first place where the hexagram's strength meets the resistance a wall implies, and a yang line occupies an even position here, a placement classical commentary reads as inherently incorrect, strength resting somewhere the system expects yielding. That mismatch may be exactly why the assault cannot be carried through: the position itself does not support the aggression the line initially attempts. Recognizing the mismatch and returning to right conduct, rather than forcing the wall regardless, is what the text rewards.
Read against the hexagram's whole arc, the fourth line is the turning point where contest gives way to correction. The rivalry that began as hidden weapons and grew into an attempted siege is abandoned here, deliberately, and the good fortune promised belongs not to victory but to the wisdom of setting the struggle aside before it is completed.