Osorkon Iv (=Biblical So, Assyrian Shilkanni) in Wikipedia
Osorkon IV was a ruler of Lower Egypt who, while not always listed as a member of the Twenty-second dynasty of Egypt, he is attested as the ruler of Tanis--and thereby one of Shoshenq V's successors. Therefore he is sometimes listed as part of the dynasty, whether for convenience or in fact.
His parentage is uncertain: he could be a son of Shoshenq V. His mother, named on an electrum headpiece in the Louvre, is Tadibast III.
Kenneth Kitchen gives his reign dates as 732/30 - 716 BC.
His reign was never recognised at Memphis where documents were dated to the reign of 24th Saite dynasty king Bakenranef. During his time, Egypt was ruled concurrently by four dynasties - 22nd, 23rd, 24th and the 25th. Shortly after Osorkon had ascended the throne, Upper Egypt was conquered by the Kushite king, Piankhi, and Osorkon IV ended ruling only the East Nile Delta region.
Relationship with Assyria
He is perhaps mentioned in the bible as the Pharaoh "So" to whom Hoshea, King of Israel appealed for help. However, So dispatched no aid or troops. The Israelite capital Samaria was captured by the Assyrian king Shalmaneser V in 722 BC and its inhabitants were imprisoned and taken to exile in Assyria and Media. To avoid military conflict with the Assyrians or even invasion, Osorkon sent presents, including several horses, to placate the new Assyrian king Sargon II, who rose to power later in 722. Osorkon's tactic apparently worked, since Sargon accepted his gifts and did not take action against him.