NG4 plot leak:
This shot from the trailer is taken from the aftermath of the last time Yakumo and Ryu clash with eachother. Yakumo uses his special powers to defeat Ryu and is deciding whether to give Ryu the death blow or spare his life. The shot is Yakumo looking at the severely cut down and battered body of Ryu. Seori begs Yakumo that he shouldn't kill Ryu, because both the dragon ninja clan and raven clan are descendants of the dark dragon and both have their role to play. She reminds Yakumo that his objective is to kill the dark dragon and with Ryu Hayabusa defeated, he can proceed and fight the dark dragon without other forces trying to interrupt and stop him. Yakumo spares Ryu as he concludes that Ryu won't get in his way anymore and continues on.
During this game it will be revealed that the dark dragon is acting as a bullwark to prevent a greater evil from entering the human world and that the demons that were allowed to enter the human world with the advent of the dark dragon were meant as obstacles for the various ninja clans to overcome, so humanity could grow stronger and one day be ready to face what lies beyond. The dark dragon also bestowed powers to the various ninja clans. The DDO and Ryu believe that the best strategy is to keep the dark dragon in a prison seal indefinitely, Ryu literally has a line in the game in which he admits that he is afraid of what lies beyond the dark dragon and that he doesn't want to risk a devestation to humanity, while Yakumo and the Raven Clan believe that the Raven Clan is strong enough to once and for all kill the dark dragon and face whatever lies beyond and exterminate that and the source of the demons so that humanity can finally be free from it all.
Yakumo proceeds to kill the dark dragon and the game ends. The DLC sets up Yakumo and Ryu as the two masters who will have to face a greater evil. The DLC is an introduction to a follow up world for a potential Ninja Gaiden 5.