
I don't think the fourth guy is a Mugo, I think it could be a rival snake god/snake demon who went after Mugo's lover and manipulated him into betraying Mugo

I do think it's Mugo. If you read again the chapter with the stairs that made Mugo's consciousness you can tell there's a fourth Mugo. Also, he's the one who told MC what to bring to avoid being eaten when he came of age (while Mugo and MC think it was Wise Mugo) and I believe that his presence was what made Mugo's scent change.
I believe he's the original, whole Mugo before whatever happened that made him split into the other personalities, and stayed hidden within the other Mugos till his lover got reincarnated and that's why the other personalities treat differently MC from other sacrifices: the Wise gave him his blood so he could avoid being devoured, the Jerk treated him unconsciously as his and the Monster thought he was the sweetest thing ever, so delicious he wants to "eat" him even if it hurts to try.
What's more, Jerk said that Monster is him so he knows what to do and that could mean they're probably beginning to bring forth their old self, the one who is all the Mugos.
At least this is what I think.

Nothing is unnecessary if the author and artist think it's hot. If noncon is their kink then y'all should respect that and stop reading if it makes you uncomfortable. It's not like this kink came out of the blue, Mugo has been non-conning/dub conning him since day 1. There are so many fluffy stories out there, if this one is too much for you then walk away and be at peace ٩(๑❛ᴗ❛๑)۶
Thank you kind translators for this note at the end, my family is from north africa and they have the same annoying and intrusive mindset as I know asian parents have too. Please everyone live your life the way you want to (๑•ㅂ•)و✧ your parents had their shot at life and they have no right to make you feel bad for your own choices and your own way of life. If they can't support you when you're doing things that make you happy then they don't want what's best for you and it's okay to take a step back and prioritize your own happiness. You're not acting spoiled or being ungrateful. You don't owe them anything other than being happy!!!