Normally, when the Meiji Restoration is considered, the "victors" are listed as Satsuma, Chōshū, and Tosa. This materialised in Satsuma and Choshu leading Imperial Japan's Army while Satsuma and Tosa commanded the Navy, with the three sharing political power.
I was, therefore, surprised to see Ryōtarō Shiba mention a fourth "victorious" domain, Higo:
... the four victorious domains at the time of the Restoration—Satsuma, Chōshū, Tosa, and Higo—...
—Shiba, 'Clouds above the Hill, Vol. 1'
What was Higo domain's relevance with respect to the Restoration and later (internal) politics?
There are no other specific mentions to the Higo domain (WP offers an alternative name as Kumamoto domain) in that series except for one, described below. WP doesn't mention anything useful on Higo or Kumamoto regarding the Restoration in the domain's article, nor in the article regarding the Restoration, nor in the one on the Bakumatsu. I tried some Google searches and those also came empty on this domain in general.
The only other hint that Shiba offers with respect to the Higo domain was when Matsunaga Masatoshi, from Higo, was assigned as Nogi Maresuke's Chief of Staff (a pretty important post) without any official training and despite advanced age:
Matsunaga was from Kumamoto Prefecture. The other fresh major generals were still in their mid-forties, but Matsunaga was already fifty-five and had no official staff training; however, he was known for his skill in combat.
—Shiba, 'Clouds above the Hill, Vol. 3'
Matsunaga fell ill and died soon after so he doesn't feature in the story for long. Yet, an (academically) unsupported rise to chief of staff cannot be the only link that a domain had to being 'victorious' in the Restoration (and there's no specific proof of this link in that quotation). So, what else is there (if anything)?
While Shiba was a writer of fiction, his works were exceptionally well researched though there are some minor errors.
As a writer of historical novels, Shiba felt obliged to respect historical facts, which were, after all, “the common property of a country’s people.” Since his novel dealt with events that many could still remember, he was at pains to make sure that he made no mistakes. “I couldn’t write anything perfunctory,” he later recalled, “so I was very nervous—it was exhausting. A novel is fundamentally fiction, but I decided absolutely not to write anything fictional about the war. . . . I could not make a mistake about the day or hour when a naval vessel or an army unit was in a particular place. It just would not do.”
Although Shiba described the novel as “historical fiction,” none of the characters is fictional, that is, entirely imagined. The book is based on extensive, and probably exhausting, research in the voluminous archives of published private memoirs, personal diaries, military and naval histories, and diplomatic records about the Russo-Japanese War. As the twenty thousand volumes in his personal library attest, Shiba was a voracious consumer of research material, sometimes spending millions of yen on his projects. It was said that once he began collecting books on a subject he quickly cornered the market. The playwright Inoue Hisashi, who was writing a play about the Russo-Japanese War while Shiba was gathering research material, discovered that none of the stores in the famous Jimbōchō used-bookstore district in Tokyo had anything left.
—Duus, 'Clouds above the Hill, Vol. 1', Introduction