John Gill Commentary 1 Kings 22:41

"And Jehoshaphat the son of Asa began to reign over Judah in the fourth year of Ahab king of Israel." — 1 Kings 22:41 (ASV)

And Jehoshaphat the son of Asa began to reign over Judah in
the fourth year of Ahab king of Israel .
] And as Ahab reigned twenty two years, (1 Kings 16:29) , Jehoshaphat must reign about eighteen years with him, and seven years after him.