The fourth line is the briefest in the hexagram and the most fortunate. Legge's translation has its subject enriching the family and gives great good fortune, with nothing qualified and nothing warned against. In a classic that hedges almost every favourable verdict it issues, an unhedged one is worth stopping on, and the reason for it turns out to be structural rather than a matter of the wealth itself.
The stroke is yielding and correctly placed in an even seat at the base of the upper trigram Xun, directly below the strong fifth line. It is the first line of the outer trigram, which is where a household meets whatever lies beyond it, and a yielding line in that position accumulates rather than asserts. Enrichment in this figure is not an achievement the fourth line claims. It is what accrues to a position that is correctly placed, adjacent to authority, and not in competition with it.
The trigram Xun is wind, and also penetration, the quality of getting in everywhere without force, and the Great Symbolism of the hexagram reads the figure as wind coming forth from fire. Set against that image, the fourth line is the station at which a household's ordering starts having effects outside its own walls. What comes back is described as wealth, and the classic gives it the largest praise anywhere in the hexagram without attaching one condition to it.