What does Genesis 49:4 mean?Jacob is offering an oracle, on his deathbed, which predicts the fates of the generations coming from his twelve sons. In the previous verse, Jacob described Reuben's lofty status as the firstborn (Genesis 49:3). Typically, this would identify Reuben as the most prestigious among his brothers. However, as Jacob explains, that position has been lost to Reuben forever.
Though details are few, what's recorded in Genesis 35:22 is disturbing. After the death of Jacob's beloved wife Rachel, Reuben apparently had sexual relations with Bilhah, one of Jacob's two servant-wives. Bilhah was something like Reuben's stepmother, as well as his father's wife. Scripture gives no reason why Reuben did this. It's possible he was trying to rebel against Jacob—to assert his dominance and take over leadership of the family in Jacob's moment of weakness.
All Genesis has revealed, so far, is that Jacob (Genesis 35:10–11) heard about this heinous act. The fact that Scripture recorded no immediate punishment does not mean there was no such reaction; but whatever happened was not included in that passage. Here, we see that this act was by no means forgotten. That sin is given as the reason Reuben lost the firstborn status.
Jacob comments that Reuben is "unstable as water." Scholars are unclear about exactly what that phrase means, but a lack of conviction marks Reuben's descendants throughout the Old Testament (Judges 5:15–16). Further, no prophets, kings, or judges will be associated with the tribe of Reuben.
This would serve as a clear warning to Israel. God holds His people accountable for sin, even when circumstances make it appear they have "gotten away with it." Also, God considers the marriage bed holy (Hebrews 13:4). To defile it as Reuben did brings serious consequences.