was a term given to four samurai

Samurai

is the term for the military nobility of pre-industrial Japan. According to translator William Scott Wilson: "In Chinese, the character 侍 was originally a verb meaning to wait upon or accompany a person in the upper ranks of society, and this is also true of the original term in Japanese, saburau...

 during the Bakumatsu

Late Tokugawa shogunate

, literally "end of the curtain", are the final years of the Edo period when the Tokugawa shogunate came to an end. It is characterized by major events occurring between 1853 and 1867 during which Japan ended its isolationist foreign policy known as sakoku and transitioned from a feudal shogunate...

 era in Japanese history

History of Japan

The history of Japan encompasses the history of the islands of Japan and the Japanese people, spanning the ancient history of the region to the modern history of Japan as a nation state. Following the last ice age, around 12,000 BC, the rich ecosystem of the Japanese Archipelago fostered human...

. The four men were Kawakami Gensai

Kawakami Gensai

was a Japanese samurai of the late Edo period. A highly skilled swordsman, he was one of the four most notable assassins of the Bakumatsu period. Gensai's high-speed sword discipline was in the Shiranui-ryū.- Birth and Early Life :...

, Kirino Toshiaki

Kirino Toshiaki

was a Japanese samurai of the late Edo period, and an Imperial Japanese Army general of the early Meiji era.-Biography:Kirino, also known as ', was renowned as one of the Four Hitokiri of the Bakumatsu. His sword style was Ko-jigen-ryū, a branch of the high-speed Jigen-ryū . Kirino's activities...

 (also known as Nakamura Hanjirō), Tanaka Shinbei, and Okada Izō

Okada Izo

was a Japanese samurai of the late Edo period, feared as one of the four most notable assassins of the Bakumatsu period. He was born in Tosa to the gōshi Okada Gihei, who had been a peasant but had bought the gōshi rank...

. They opposed the Tokugawa shogunate

Tokugawa shogunate

The Tokugawa shogunate, also known as the and the , was a feudal regime of Japan established by Tokugawa Ieyasu and ruled by the shoguns of the Tokugawa family. This period is known as the Edo period and gets its name from the capital city, Edo, which is now called Tokyo, after the name was...

 (and later, supported the Meiji Emperor). These four samurai were warrior elite and widely considered undefeatable by normal people. The word hitokiri literally means "manslayer."