How many kings did Isaiah serve as a prophet?

Isaiah served under 4 kings as a prophet. In his notes on Isaiah in his Study Bible, John MacArthur says:-

Isaiah, son of Amoz, ministered in and around Jerusalem as a prophet to Judah during the reigns of 4 kings of Judah: Uzziah (called "Azariah" in 2 Kings), Jotham, Ahaz, and Hezekiah (1:1) from ca. 739-686BC.