Nipping is not necessarily bullying. Nipping is a gesture of power from a more dominant piggy. It doesn’t hurt and doesn’t break the skin.
On its own is not a concerning behaviour.
If you are seeing other concerning things which may mean their bond is in trouble, then do give as much detail as possible and we can help further.
If you are seeing an increase in dominant behaviour, behaving as if in season all the time in a sow over two years old, then a vet check for ovarian cysts would be a good idea.
But in answer to your question, no, adding a fourth piggy would not resolve any issues that are already present in an existing herd - If one original sow is being outcast, then adding a fourth would not change the way they feel about that original sow, she would still be disliked. It may even make things worse. There is also no guarantee a fourth piggy would be accepted by the dominant piggy in any event. You don’t say their ages, but older sows get, the less willing they can become in accept any change or any newcomers into their herd.
Adding a single sow to a bonded pair can cause an outsider situation to occur - the third newcomer may never bond quite as well as the original two were bonded to each other.
If there is any issue in the bond between the piggies you have, then the only option really is to split them up. You could then get a new friend for the piggy left single so you make two separate pairs.
I have added a guide below to help you determine whether your sows are in a tense situation with a problematic bond
Bonds In Trouble
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
Sows: Behaviour and Female Health Problems (including mounting and ovarian cysts)
Dominance Behaviours In Guinea Pigs
