The fourth line is where the hexagram becomes courtly and, in the same gesture, unceremonious. Legge's translation describes a cup of spirits and a vessel of grain, with earthenware used alongside them, introduced through the window, and the verdict is that in the end there will be no error. Every detail is a reduction of the proper form. Bronze would have been the correct material and the door the correct route, and a vassal presenting himself to his ruler in ordinary times would have used both.
Those details belong to the Zhou court and they carry the reading. Earthenware in place of bronze, and a window in place of a door, describe a relationship maintained under conditions that make the usual observances impossible, and the text approves of it. The commentary on the image locates the point at the meeting of those above and those below. What has to survive a time of danger is the connection itself, not the apparatus normally used to express it.
The stroke is yielding in an even place, correct by the ordinary rule, standing at the base of the upper trigram Kan directly beneath the strong fifth line. That adjacency is the whole of its situation. This line has no strength of its own and does not need any, because what it is doing is reaching the one firm thing above it by the only route still open, and the text is satisfied that a plain offering delivered awkwardly is worth more here than a correct one that cannot be delivered at all.