The fourth line goes inside the darkness and comes out again. Legge's translation has the subject entering the left side of the belly, reaching the very mind of the darkening of the light, and then quitting the gate and the courtyard. It is the only line in the figure that treats understanding as a place a person travels into, and the sequence matters more than the image: he gets in, he learns what the thing actually wants, and he leaves.
The stroke is yielding in an even place and correctly placed at the base of the upper trigram Kun, which is to say it has just crossed out of the light and into the earth that covers it. That crossing is the whole of its situation. A line at this station is close enough to the centre of the darkening to see how it is organised and not yet held by it, and yielding is the quality that makes the approach survivable, because nothing about this position could be forced.
The gate and the courtyard are the boundary of a household in the language of the period, so leaving them is a departure from a place one belonged to rather than an escape from custody. What the line commends is neither infiltration nor endurance. It is the recognition that once the intent of a corrupt arrangement is genuinely understood, staying inside it serves no further purpose, and that the person who understands it is the person best placed to know so.