Nine in the fourth place is a warning aimed at somebody who is doing well. The line grants that the subject may be entirely firm and correct and still calls the position dangerous, which makes it one of the few readings in the classic where good conduct is explicitly not enough on its own. The commentary locates the fault in the place rather than in the person: it is not the one appropriate for him.
A querent drawing this has usually advanced faster than the standing that would justify it, and the marmot in the image describes the resulting behaviour rather than the person's character. Advance held nervously, with the gains hoarded and the movement kept quiet, is what the text is picturing. In a hexagram whose whole figure is the sun coming out over the earth, an advance conducted out of sight is already at odds with the situation it sits inside.
When the line changes, the upper trigram Li becomes Gen and the figure becomes Splitting Apart, King Wen 23, in which five yielding lines have undermined a single strong one at the top and the structure is coming away. That is the danger this line names, drawn out to its conclusion. What is held without the position to hold it gets eroded from underneath, and the classic files that as the end of advancing into ground which was never one's own.