The fourth line opens the second fire and does it violently. Legge's translation exclaims at the manner of the subject's coming, how abrupt it is, with fire, with death, and with being cast aside. The commentary on the image supplies the reason in a few words: none can bear with him. Where the hexagram's other lines describe brightness that attaches to something and therefore lasts, this one describes brightness that arrives all at once and is finished by its own arrival.
The image is a sequence rather than a single picture. Something flares, it consumes what it was burning, and what remains of it is discarded. A doubled Li figure would ordinarily read the passage from the lower trigram to the upper as the light being handed on, and the fourth line is where that handover happens. The text shows it botched, with the second flame taking hold in a way that leaves nothing underneath for it to stand on.
The stroke is strong in an even seat, incorrect by the ordinary rule, and it sits directly beneath the yielding ruler of the upper trigram. Excess force in a place that calls for yielding is the structural fact behind every part of the image. Fire needs fuel and a person needs a position others will tolerate, and a line carrying more strength than its seat can hold destroys both at the moment it appears.