The fourth line is where retreat becomes a decision rather than a necessity. Legge's translation has the subject retiring notwithstanding his likings, and states that this brings good fortune in a superior man while a small man cannot attain to it. Nothing external has changed between this line and the one below. The situation still holds whatever the person wants, and the person goes anyway, which is the entire content of the line.
Superior man and small man are the classic's own pair, a distinction of character drawn in the social vocabulary of the period, and the line is given in those terms here because that is how the text gives it. The operative claim underneath the vocabulary is about capacity rather than rank. Somebody who can act against their own preference has a resource that somebody governed by preference does not, and the text says plainly that the second sort will not manage this particular move.
The stroke is strong in an even place and stands at the base of the upper trigram Qian, the first line in the figure to have cleared the mountain and entered the open sky above it. That is the structural reason the withdrawal has become voluntary. Below, in Gen, retreat was obstructed and the whole difficulty lay in getting out at all. Here the way is open and the only thing holding the subject is that he would rather stay, which is a smaller obstacle and a harder one.