The fourth line takes the damage off the furniture and puts it onto a body. Legge's translation shows the couch already overthrown and the injury going on to the skin of the one who lies on it, with the verdict of evil. Through the first two lines the loss was measured in legs and frame, things that can be replaced. Here the hexagram crosses a boundary it does not cross back over, and what is being stripped can no longer be repaired by carpentry.
The imagery has been careful to earn this. A couch is where a person is at their least defended, and the hexagram has spent three lines removing everything between that person and the erosion beneath them. The verdict needs no qualification at this point, and the text supplies none, because the situation has passed out of the range where advice about conduct or timing would make any difference.
A yielding stroke correctly occupies this even position at the base of the upper trigram Gen, and the correctness is worth naming precisely because it changes nothing. As at the second line, propriety in placement does not slow a decline that has nothing firm standing against it, and this stroke merely carries the process across into the upper half of the hexagram. What follows is the recovery worked by the fifth and sixth lines, but the fourth is where Splitting Apart admits how much has already been lost.