Usermaatre Osorkon IV was an ancient Egyptian pharaoh during the late Third Intermediate Period. Traditionally considered the last king of the 22nd Dynasty, he was de facto little more than ruler in Tanis and Bubastis, in Lower Egypt. He is generally – though not universally – identified with the King Shilkanni mentioned by Assyrian sources, and with the biblical So, King of Egypt mentioned in the second Books of Kings (17:4).

The Third Intermediate Period of ancient Egypt began with the death of Pharaoh Ramesses XI in 1077 BC, which ended the New Kingdom, and was eventually followed by the Late Period. Various points are offered as the beginning for the latter era, though it is most often regarded as dating from the foundation of the Twenty-Sixth Dynasty by Psamtik I in 664 BC, following the departure of the Nubian Kushite rulers of the Twenty-fifth Dynasty after they were driven out by the Assyrians under King Ashurbanipal. The use of the term "Third Intermediate Period", based on the analogy of the well-known First and Second Intermediate Periods, was popular by 1978, when British Egyptologist Kenneth Kitchen used the term for the title of his book on the period. While Kitchen argued that the period was 'far from being chaotic' and hoped that his work would lead to the abolishment of the term, with his own preference being the 'Post-Imperial epoch', his use of the term as a title seems only to have entrenched the use of the term.