Your little girl is getting older and more aware; thus the questions. She's now mature enough to notice the differences. I suggest she is asking questions because she wants to learn and they may not have much to do with feelings about her personal situation. She's curious. I love the way kids are honest and direct with their questions. If she doesn't appear upset I would only focus on the question and not why she might be asking it. You can ask her why she's asking and thus find out as the joke goes that all the child wanted to know whether they came from Oregon or California.
Be honest using words she'll understand and only including the information that she is able to understand. I like Soccermomof1's answer for how her son got here. You can say, you grew in Mommy's tummy but is she really wanting to know the facts of her birth or why she's here?
For statements such as I want my Mommy and Daddy, sympathize with her. You can say something like you'd like Mommy and Daddy to live together? If she says yes, tell her that would be great! At four it's enough to say something simple such as we don't live together but we both love you very much.
Remember she is four and thinks in very concrete ways. She is unaware of all the emotion and complications of a relationship. She knows she loves Mommy and Daddy; that other mommies and daddies live together and she wants hers to live together too. There are many good books about different families.
I adopted a foster daughter and she loved one of the books I found about different families. She's 30 now, with two children who have different fathers. We talk a lot about how families are different. My granddaughter calls the daughter of one of her mother's friends her sister. She started calling her sister when she was 4 or 5. When she was older she sometimes would add, "well, she's not really my sister, except she is, in a different way."
I say just relax and not worry about how you answer her questions. Give a simple easily understood answer. If she wants more information, she'll ask more questions. Be open to questions. Emphasize love. And most of all don't over think the question or the answer.