Comentario sobre Números 14:18

ה' אֶרֶךְ אַפַּיִם “The Lord is long suffering” towards both the righteous and the wicked. When Moses ascended to Heaven to receive the Law he found the Holy One, blessed be He, engaged in writing: “The Lord is long-suffering”. He asked, “Surely only to the righteous?” The Holy One, blessed be He, answered him, “To the wicked also!” Whereupon Moses said: “The wicked — let them perish!” He (the Lord) replied to him: “I swear by your life that you shall eventually need this thing (the extension of My mercy also to the wicked)”. When the Israelites sinned at the incident of the Golden Calf and at that of the spies, Moses offered prayer before Him, with mention of God’s attribute of ארך אפים (of His being long suffering). The Holy One, blessed be He, then said to him: Did you not say, “Surely only to the righteous!?” Whereupon Moses replied, “But did You not tell me, ‘To the wicked also’?” Let then — Moses added — the strength of My Lord be great [even as Thou didst say] — by fulfilling Thy statement and not mine! (Sanhedrin 111a).

פוקד 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.

ה׳ ארך אפים, The Lord is slow to anger, etc. Why did Moses not invoke all of the thirteen attributes G'd had taught him after the sin of the golden calf? We may understand this in accordance with Rosh Hashanah 17. We quote: "ה׳, ה, I am the Lord before man sins as well as after he has sinned and has repented his sin." In my commentary on Exodus 34,6 I have quoted what the Rosh has to say on this subject as well as my own understanding of it. You have learned that the attribute of Mercy has to be invoked on behalf of man even before he has sinned. In this instance Moses had to invoke that attribute only after the sin. This is why he said השם only once.