How do I get past level 4 (green, sewer)? I’m stuck in the labyrinth and can’t find the key...

Hey there. This part got me stuck too. I would tell you how to solve it, but I genuinely couldn't remember the exact path I took. 

Rather, I decided to record the path I took that leads to the key. I hope this video is able to help you. 

Ralsei's Nikki: Sewer Maze - YouTube