You replaced upper moon 4 because he died during the swordsman village arc,The Demon slayers paired up with Lady Tamayo,they created a potion to make Muzan extremely old to atleast slow him down,she did messed up and he turned into a BABY.

Muzan:WTF HAPPENED TO ME?!

Kokushibo:Wtf-

Douma: Awwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww how cute!

Baby Muzan:Shut up!

Akaza:Uh-