Responses
Because in Chapter 10, just before Jeon fell of the roof, he is thinking of which Mugo is on his side. You see a staircase where all three are standing and at the bottom of it there is a fourth one in the shadow from which you only see the shiny eyes.
It is also a pretty common trop in story talking about Dissociative Identity Disorder to have one that is still hidden that happens to be the dangerous one.
Yes, thats why a lot of people think the same. So it shouldn’t be surprising if a fourth one comes out.
I dont know how people are getting there are four..red pane had a stutter and is the monster we already met. They even stated earlier already that there are just three of them. Monster Mugo is the one who eats humans and requires sacrifices.