Thanks for the clarification on why you and Karma will be involved in the discussion. I honestly don't think it's necessary for the two of you to be there in order for the daughter to know she can come to you.
My further question would be, since you wish as a child you'd had more concrete answers, what concrete answers are you thinking of giving a 4 year old? To tell her directly this is Karma's girlfriend? What does 'girlfriend' mean to this 4 year old? When she asks, do you intend to go on to tell her it means the two of them have sex?
I recently read a short excerpt about a father on a train, whose young daughter asked a question that was beyond her years by far. When they got off the train, he asked her to carry his work bag. She protested that it was too heavy for her. He said, "For now, so is the answer to your question. Let me carry that for a bit, too, until you're older." (From a Corrie ten Boom book in case anyone is interested.)
I get the feeling there's almost an eagerness to Discuss Poly with this little girl; an eagerness on the part of you, karma, your brother, and his wife, to Be Open-Minded and Teach Open-Mindedness. My feeling is that no matter how precocious a 4 year old is, no matter how intelligent, she is still a very young child. Sometimes, as adults, we can say, "She's a friend," and "We'll talk about that when you're older." I work with kids, and have a great deal of experience with kids this age. Take what you want from my thoughts.