CCG writes – “9-nine-:Episode 4 takes place in a parallel universe with a different chain of events. Like the other timelines, powerful artifacts have made their way into the hands of students who use their newfound abilities for good or evil. When you first begin this story youll be chatting with Sophie and be given an overworld view of the various timeline branches (Episodes 1-3). Its highly recommended that youre familiar with the previous three games before playing this one. Youll be doing a lot of timeline hopping in the hopes of defeating Iris and saving the lives of your friends.”
Source: N4G PC 9-nine-: Episode 4 review – ChristCenteredGamer