John Gill Commentary 2 Kings 15:12

"This was the word of Jehovah which he spake unto Jehu, saying, Thy sons to the fourth generation shall sit upon the throne of Israel. And so it came to pass." — 2 Kings 15:12 (ASV)

This was the word of the Lord which he spoke to Jehu
Which was now fulfilled in the short reign of Zachariah:

saying, your sons shall sit on the throne of Israel to the fourth
generation ;
see (2 Kings 10:30) , and so it came to pass; as every word of the Lord does, not one fails; for after Jehu reigned Jehoahaz, Jehoash, Jeroboam the second, and Zachariah, all descendants of Jehu.