The fourth line holds the one animal in the figure and it is not a flattering one. Legge's translation shows the subject advancing like a marmot, a burrowing rodent, and states that however firm and correct he may be his position is perilous, while the commentary on the image gives the reason plainly: the place is not the one appropriate for him. The verdict is unusual in that correctness is granted and then set aside as insufficient.
The structure is what the commentary is pointing at. This is a strong stroke in an even place, incorrect by the ordinary rule, and it is the first line of the upper trigram Li, the bright one, standing immediately below the yielding fifth line that rules the hexagram. A firm line pressed up against the ruling seat and not entitled to the ground it stands on is the classic picture of somebody who has advanced past what their position can support.
The rodent supplies the manner rather than the fact. It is an animal that gathers and hides and moves nervously, and the advance the line describes has the same quality: acquisitive, anxious, unwilling to be seen doing it. Progress in this hexagram is the sun coming up over the earth, an advance that is visible by its nature, and the fourth line is what advance looks like when it wants the gains without the light.