[9] In 1894, Frederick Dickins recorded in English the existence of "four Tokugawa guardians" during Sengoku period, although he did not name these individuals.
[10] After the peace negotiations between Tokugawa Ieyasu and Toyotomi Hideyoshi following the battle of Komaki and Nagakute, Tadatsugu, Naomasa, Tadakatsu, and Yasumasa gained fame in Kyoto.
There are several theories regarding this: Regardless the version, Yū Kawamura from Chiba University saw the step to place of most military effective Tokugawa vassals in control of those regions were to pacify the populations of newly subdued territory which formerly ruled by the Hōjō clan before the Siege of Odawara (1590), while also guard the eastern domains from any influence or threat from the Satomi clan which has not yet submit to Toyotomi rule at that time.
[12][13] Stephen Turnbull stated that prior to the inclusion of Ii Naomasa, the Tokugawa-shitennō consisted of Ishikawa Kazumasa, Sakai Tadatsugu, Sakakibara Yasumasa, and Honda Tadakatsu, as they fought in the battle of Anegawa.
[14] After Battle of Sekigahara however, Ieyasu seems to have disproportionate attitude towards the Fudai daimyo vassals, as it seems he rewarded Tozama daimyō, newcomers who just entered Ieyasu service during Sekigahara Campaign such as Ikeda Terumasa, with far bigger reward of domains increase than his hereditary Fudai vassals like Yasumasa, Tadakatsu, or Naomasa.