John Gill Commentary 2 Kings 14:21

"And all the people of Judah took Azariah, who was sixteen years old, and made him king in the room of his father Amaziah." — 2 Kings 14:21 (ASV)

And all the people of Judah took Azariah
Called Uzziah in the next chapter, and so in the book of Chronicles; both names signifying much the same, the one "the help of the Lord", the other "the strength of the Lord":

(which was sixteen years old; )
yet as he began to reign in the twenty seventh year of Jeroboam, (2 Kings 15:1) and Jeroboam began to reign in the fifteenth year of Amaziah, (2 Kings 14:23) , he could be but four years of age, for the solution of which, (See Gill on 2 Kings 15:1):

and made him king instead of his father Amaziah ;
which was after his death, and not when he fled to Lachish, as Kimchi thinks.