The fourth line gives Biting Through its hardest mouthful and its best reward. Legge's translation describes gnawing flesh dried on the bone and obtaining the pledges of metal and arrows, and counsels that it will be advantageous to recognise the difficulty of the task and remain firm, in which case there is good fortune. The obstruction here is the most stubborn in the hexagram, meat that has dried onto bone and will not separate, and what is recovered from it is not soft matter at all but something straight and hard.
The arrow supplies the sense of the reward. An arrow is straight, and metal does not bend under pressure, so the pledge taken out of the difficulty is a commitment that will hold. Nothing about this line suggests the work is quick. Its promise is that the work is worth doing, which is a different promise from the effortless bite of the second line, and a better one.
This yang stroke opens the upper trigram Li, fire and clarity, from an even position that classical commentary treats as incorrect for a firm line. The mismatch is why the text adds its condition rather than simply promising good fortune. Strength placed where yielding is expected can force a matter and lose it; the instruction to recognise the difficulty first is what keeps that strength deliberate. Good fortune here is earned by naming the size of the task before starting, and then not letting go of it.