[1] His full baptismal name was Siaosi Tāufaʻāhau Tupoulahi, but he was soon better known by the traditional title Tupoutoʻa, which was bestowed upon him in 1935 and subsequently became reserved for crown princes of Tonga.

By consequence, the King's daughter, Pilolevu, was the first Tongan woman to descend from the bloodlines of the three major royal dynasties and become the highest-ranking person ever.

His coronation took place on 4 July 1967, his 49th birthday, in Nukuʻalofa, with dignitaries including the Duke of Kent and New Zealand Prime Minister Keith Holyoake in attendance.

"[14] During the 1980s, his government adopted a tone of appeasement towards France in its Pacific nuclear tests at Moruroa, which he visited at the invitation of Gaston Flosse.

[17][18] In 2004, he was named a press freedom predator by Reporters Without Borders, a move which was criticised by the owner of an independent newspaper in Tonga.

[20] On 15 August 2006, Tongan Prime Minister Feleti Sevele interrupted radio and television broadcasts to announce the king was gravely ill in the Mercy Hospital in Auckland and to ask the 104,000 people of the island chain to pray for their monarch.

[23][24] He was succeeded by his eldest son, George Tupou V.[25][26] The king was buried on 19 September 2006 at Malaʻekula, the royal cemetery in Tongatapu.

[27][28][29] Mourners included foreign dignitaries from 23 countries, including Japanese crown prince Naruhito; Australian governor-general Michael Jeffery; New Zealand governor-general Dame Silvia Cartwright and prime minister Helen Clark; Fijian vice-president Ratu Joni Madraiwiwi and prime minister Laisenia Qarase; Vanuatu president Kalkot Mataskelekele; governor of American Samoa Togiola Tulafono; Niue premier Young Vivian; president of French Polynesia Oscar Temaru; and the Duke of Gloucester, a cousin of Queen Elizabeth II.

[28][29][30][31] He married a distant relative, Halaevalu Mataʻaho ʻAhomeʻe (1926–2017), on 10 June 1947, during a double nuptial ceremony with his brother Prince Fatafehi Tuʻipelehake.