Sforno on Numbers 14:18/ספורנו על במדבר י״ד:י״ח

1פוקד עון אבות על בנים, this may extend until the fourth generation, i.e. if the fourth generation of the original sinner will still continue in the wicked ways of their forbears that will make the measure of sin of those families in the third generation so full that it has become irreversible and G’d will take the appropriate action. If the sons and grandsons continued in the evil ways of their forbears but not until the fourth generation, G’d will suspend retribution to see how the fourth generation would conduct themselves before exacting retribution.