Albert Barnes Commentary 2 Kings 17:4

"And the king of Assyria found conspiracy in Hoshea; for he had sent messengers to So king of Egypt, and offered no tribute to the king of Assyria, as he had done year by year: therefore the king of Assyria shut him up, and bound him in prison." — 2 Kings 17:4 (ASV)

The "So, king of Egypt," mentioned here is generally identified with Shebek (c. 730 B.C.), who is the same figure Herodotus calls Sabaco. King Hoshea’s appeal to him was a return to a policy that had been successful in the reign of Jeroboam I (1 Kings 12:20) but had not been used by any other Israelite monarch since.

For many years, Egypt had been weak. Sabaco, however, was a conqueror who, leading Ethiopian armies, had invaded and taken control of the country. In his own inscriptions, Shebek boasts of receiving tribute from “the king of Shara” (Syria), which is likely his way of noting Hoshea’s appeal for an alliance.

References to Israel's leanings toward Egypt are frequent in the prophecies of Hosea (Hosea 7:11; 11:1, 5; Hosea 12:4). At the same time he was accepted as a vassal by Sabaco, King Hoshea stopped paying tribute to Shalmaneser. This act of open rebellion provoked the punishment that followed.