A rare divine promise granted unprecedented stability to Jehu in the Northern Kingdom, rewarding him for carrying out God's will. Because Jehu was not a prophet, this communication was not a direct revelation [רלב״ג]. Instead, the divine message was delivered through a prophetic messenger [מצודת דוד, רד״ק, ביאור שטיינזלץ], with some identifying this messenger specifically as Jonah the son of Amittai [רש״י, רד״ק].
God praised Jehu for acting in the best possible manner, having executed the destruction of the house of Ahab with absolute precision, exactly as God intended [מצודת דוד, ביאור שטיינזלץ]. As a reward for this strict obedience, Jehu was promised that four generations of his descendants would succeed him on the throne [מצודת דוד]. This promise was fully realized, establishing the only dynasty in the history of the Northern Kingdom to endure for four consecutive generations [ביאור שטיינזלץ], passing through Jehoahaz, Jehoash, Jeroboam, and Zechariah [רד״ק].
The choice to reward Jehu with exactly four generations was not random. The primary approach among commentators is that this reflects a principle of measure for measure. Since Jehu destroyed the Omri dynasty, which consisted of four generations of kings (Omri, Ahab, Ahaziah, and Joram), he earned four generations for his own lineage [רש״י, רד״ק, חומת אנך]. Alternatively, an approach based on Midrash suggests the number four corresponds to the four distinct targets Jehu eliminated: Joram king of Israel, Ahaziah king of Judah, the entire house of Ahab, and the worshippers of Baal [חומת אנך].
Alongside the reward, this promise carried a severe moral demand. Although Jehu successfully eradicated Baal worship, he chose to leave intact the golden calves originally set up by Jeroboam [רד״ק]. Jeroboam had initially built these idols out of political fear, worried that if the people traveled to Jerusalem, the monarchy would revert to the house of David. However, once Jehu received a clear divine guarantee that his descendants would sit securely on the throne for four generations, he lost this excuse completely. With his rule guaranteed, he had no reason to fear losing power. His decision to cling to Jeroboam's sins, despite the political security granted to him, provoked God's anger. Yet, because the promise of a lasting dynasty had already been made, God could not immediately unleash His wrath upon him [מלבי״ם].