Grand Duke William IV remains a relatively unknown figure in Luxembourg's history, despite having played a crucial role in consolidating the national dynasty. His reign, which lasted just over six years, was overshadowed by the progression of an illness that struck him in 1898, gradually plunging him into aphasia. However, despite his fragile health, William IV left a significant legacy, notably through his decision to confirm his daughter, Marie-Adélaïde, as heir to the throne, thus ensuring the continuity of the House of Nassau-Weilburg.

Prince William's early years were closely linked to the Duchy of Nassau, where his ancestors had reigned for nearly nine centuries. His education, entrusted to private tutors, was aimed at preparing him for military life and his future princely responsibilities. From 1858, he studied at Biebrich Castle under the supervision of Professor Philipp Türck. He took courses in ancient and modern languages, mathematics, history, literature and religion, and developed a marked spiritual sensitivity under the guidance of the ecclesiastical advisor Dilthey. Lessons in drawing, music, dance and etiquette completed his education, which included a military component from 1863 onwards.

A lively and curious child, William studied with Adolphe Goetz, son of counsellor Friedrich Goetz, from 1862 onwards and stayed with him in the Rheingau and at the Platte hunting lodge, owned by the ducal family. At the age of eleven, he met Emperor Franz Joseph I of Austria and in 1864 gave a highly acclaimed speech on the 25th anniversary of his father's reign. His early years were also punctuated by visits to his uncle, Prince Nikolaus, and excursions to the Wiesbaden region.

The year 1866 marked a turning point in the life of Prince William, then aged 14. Duke Adolphe, true to his convictions, supported Austria in its conflict with Prussia, but after Austria's defeat at the Battle of Königgrätz, Prussia annexed the Duchy of Nassau and Adolphe lost his throne. The ducal family was then forced to leave its historic stronghold.

After an initial stay in Heidelberg and a stop in Rumpenheim, the family spent the spring and summer of 1867 partly in Ouchy, near Lausanne, and partly in Gräfenberg, where William celebrated his confirmation on 7 July 1867.

Deprived of his dynastic claims to the Duchy of Nassau, Prince William chose a military career. In September 1867, he entered the cadet school of the Kingdom of Saxony in Dresden. After graduating, he stayed in Switzerland to prepare for his officer's exam, which he passed with honours on 17 October 1871 in Brünn. He then joined the Austrian army as a lieutenant in the 12th Dragoon Regiment ‘Graf Neipperg’ in Proßnitz (Prostějov in the Czech Republic).

He then served in various garrisons of the Austrian monarchy, was promoted to first lieutenant in 1873, and then to captain in 1875. Prince William, who took part in numerous military manoeuvres, was highly regarded by his superiors and comrades for his skills, seriousness and courtesy. He gradually rose through the ranks, becoming major in 1878, lieutenant colonel in 1881, then colonel in 1884, a rank that earned him command of the 1st Hussar Regiment ‘Kaiser’.

In 1884, the death of Prince Alexander of Orange-Nassau, heir apparent to King Grand Duke William III, increased the likelihood that his father, Adolphe, would ascend to the throne of Luxembourg. Prince William then actively prepared for his role as Crown Prince. He left his military duties and accompanied his father on his first visit to Luxembourg in 1889, during his father's regency. He returned in November 1890, when Adolphe ascended the throne, and Paul Eyschen, head of the Luxembourg government, introduced him to the day-to-day affairs of the country.

In 1884, Duke Adolphe refused to give his consent, citing the difference in religion between Prince William, a Protestant, and Princess Maria Anna, a Catholic from the exiled Portuguese royal dynasty, the Braganza family. However, Maria Anna's family had ties with the Luxembourg clergy, and it is likely that the Bishop of Luxembourg, Koppes, was aware of these marriage plans.

In 1890, Crown Prince William sought Paul Eyschen's intervention with the Holy See in order to obtain the blessing of Pope Leo XIII, but to no avail. The Pope refused to make an exception to the rule and demanded that any children born of this union be raised in the Catholic faith. Although William was willing to accept this condition, his father, who was attached to the Protestant tradition of the Nassau family, remained reluctant. It was not until 1893 that Grand Duke Adolphe finally gave his consent to his son's marriage to Princess Maria Anna.

The wedding was celebrated at Fischhorn Castle in Zell am See on 21 June 1893, according to both Catholic rites and Protestant customs. The birth of their eldest daughter, Princess Marie-Adélaïde, symbolised the religious transition of the Luxembourg dynasty, as she was baptised according to Catholic tradition. 

The couple had six daughters: Princess Marie-Adélaïde (1894–1924), who succeeded her father; Princess Charlotte (1896–1985), who succeeded her sister; Princess Hilda (1897–1979), Princess Antonia (1899–1954), Princess Elisabeth (1901–1950) and Princess Sophie (1902–1941).

In 1898, while staying at the Grand Ducal Palace, Grand Duke Adolphe slipped and fractured his femur. Upon hearing the news, William suffered a stroke, from which he seemed to recover fairly quickly. It was not until 1905, the year he ascended to the Grand Ducal throne, that the effects of this stroke became apparent. While William IV was staying at Hohenburg Castle in Bavaria, he suffered another stroke, which marked the beginning of progressive paralysis.

His health deteriorated rapidly and he gradually became unable to perform his duties. The Grand Ducal family rarely resided in Luxembourg at that time, as the Grand Duke had to undergo treatments that kept him away from his adopted country. In 1908, his wife Maria Anna was sworn in as regent of the Grand Duchy, ensuring the continuity of power. During this time, the Grand Ducal administration rented Villa Schaefer so that the regent could reside near the Grand Ducal Palace with her daughters, as the former Berg Castle, the Luxembourg residence of Prince William and Princess Maria Anna before William IV's accession to the throne, was undergoing reconstruction.

In the absence of a male heir, the succession of Grand Duke William IV, the last agnate of the Nassau family, was contested. Thus, in 1907, a cousin from the morganatic marriage of Adolphe's brother, Nikolaus de Nassau, to the daughter of the Russian poet Pushkin reserved the right to claim the Grand Ducal throne and the family inheritance. He even received the support of certain liberal-socialist members of parliament, before William IV confirmed the legitimacy of Princess Marie-Adélaïde by adding an amendment to the Family Pact. With this gesture, he abolished the Salic law of succession.

Struck by aphasia for seven years, Grand Duke William IV had no respite and died on 25 February 1912, five months after his return to Berg Castle, where reconstruction work had been completed. His remains were then transferred to the crypt of the Protestant church in Weilburg, where his Nassau ancestors already lay.