Or HaChaim on Exodus 20:5/אור החיים על שמות כ׳:ה׳

1 פקד עון אבות, "Who remembers the sins of the fathers, etc." Having stated that G'd is "jealous," and knowing that in our world anyone who is jealous reacts promptly against the person he is jealous of, G'd might have been expected to react similarly. We know from many thousands of years of history, that people who have given G'd ample reason to be "jealous" appear to enjoy an undisturbed life. Where then was G'd's "jealousy?" Are we, G'd forbid, to assume that G'd was either unaware of these sinners or unable to punish them? The Torah therefore states that if we observe the very opposite of jealousy in G'd's apparent relationship to these sinners, the reason is that though it is His nature to be "jealous," He is also פקד, remembers things, i.e. He does not exact retribution immediately. His patience may extend for up to the fourth generation after the sins committed by the original sinner. G'd vows that His patience does not extend beyond the fourth generation. The reason why G'd practices this patience is that if He were to punish everybody immediately mankind would cease to exist. This is the only reason that G'd appears to turn a blind eye to the sins committed by people. He waits until the sinners' children grow up hoping that these children will pursue a lifestyle more to His liking. When the children of sinners do this it has a positive effect on their parents' standing in the hereafter (Sanhedrin 104). Should the children of the sinner continue the evil ways of their fathers, G'd considers this when He punishes the second generation; or, He may wait for an improvement of the third generation before He includes the sins of the fathers when He punishes that generation for its own sins. He will not defer punishment past the third generation as by that time the family of the original sinner has become so deeply rooted in the domain of the קליפה that there is no hope for rehabilitation by their own efforts. When G'd's "jealousy" finally is displayed against such a family it is much more destructive than had it been displayed already against the original sinner. On no account must we assume that G'd makes a rule of deferring punishment until the fourth generation. As far as G'd's timetable is concerned even punishment meted out in the fourth generation is considered "immediate." Three generations are considered three hundred years, whereas a day in G'd's terms is one thousand years in our terms. This is implied in the Torah saying: על, ועל instead of merely saying that G'd's patience would extend עד דור רביעי, until the fourth generation as the Torah says concerning the reward which will be paid לאלפים "up to two thousand generations."

2The concept of G'd not displaying His jealousy against the sinners is something very confusing, especially so when such sinners keep piling new sins upon old ones and enjoy success (in their terms). Once one comprehends the fact that G'd may limit such success to four generations the fact that G'd sometimes appears to punish a particular family who do not appear to have been especially sinful becomes acceptable. This is what Assaph had in mind in Psalms 73,17 when he said "I will understand this in light of their eventual fate." He referred to people who were at the tail end of four generations of sinners and now experienced G'd's "jealousy." Whereas G'd never includes sins committed five generations ago in the punishment of a particular generation, the reverse is true of reward. Reward for good deeds may be extended for up to two thousand generations.