Okay, after successfully (mostly) bonding Cheddar, Poopy, and Meatball, I go and decide to get another bunny as Meatball's girlfriend, Noodle. My former trio lived in the same cage and were fine.
Cheddar and Poopy are as bonded as can be and are in the same cage now. They are enclosed in a gate. Meatball and Noodle are in another cage together. They are not bonded, but they get along very well. Meatball and Noodle get free roam of the living room except for the caged off part.
Now that Poopy doesn't live with Meatball anymore, she doesn't like him anymore. It's not like she ever liked him that much, but they were okay. She keeps nipping him through the gate.
Poopy hates Noodle. She stalks her and bites her through the gate all the time. Poopy also marks with poops everywhere. Cheddar seems to not like Noodle either.
How should I go about bonding the four? Any tips from people who have successfully bonded four buns?