Exodus 34:4-9 (King James Version)
King James Version (KJV)
Public Domain
[A Public Domain Bible] [KJV at Zondervan] [Zondervan]
4And he hewed two tables of stone like unto the first; and Moses rose up early in the morning, and went up unto mount Sinai, as the LORD had commanded him, and took in his hand the two tables of stone.
5And the LORD descended in the cloud, and stood with him there, and proclaimed the name of the LORD.
6And the LORD passed by before him, and proclaimed, The LORD, The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth,
7Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear the guilty; visiting the iniquity of the fathers upon the children, and upon the children’s children, unto the third and to the fourth generation.
8And Moses made haste, and bowed his head toward the earth, and worshipped.
9And he said, If now I have found grace in thy sight, O LORD, let my LORD, I pray thee, go among us; for it is a stiffnecked people; and pardon our iniquity and our sin, and take us for thine inheritance
Having said that, I don't see anywhere in this verse where, if one asks forgiveness for generations older than the third or fourth, that it is a sin or would prove that someone is 'dead in their sins', which is what you originally claimed.
Further, the verses I posted to you don't say anything about the prophets asking God to forgive the sins of their fathers "back to the third or fourth generation, but I know you won't forgive the sins of the fathers in older generations."
So, nice try, but you are still wrong.