Not a typo
[Update: Koei Tecmo have since tweeted out confirmation that Warriors Orochi 4 will also be heading West this year.]
Koei Tecmo have announced that Warriors Orochi 4 is happening, and is coming to Japan this year. The announcement was made via a teaser video, which you can check out below.
The video itself names the game “Warriors Orochi 3” as Western localisations of the series are titled “one game ahead”, so to speak. This is due to the re-naming of 2008’s Musou Orochi: Rebirth of the Demon Lord, which is known as Warriors Orochi 2 in North America.
No further info was forthcoming about the new game, including localisation possibilities or even platforms. Warriors Orochi 4 launches in Japan sometime in 2018.