N.G.
Why should you have to explain something like that to a small child??? That is nothing that your friend should be having a conversation with a four-year-old about. I would say something like "When God wants us to have another child, we will." or, if your friend is not a believer, just say "Everything happens for a reason, and when we are supposed to, we will." Or even just something as simple as, "Our family isn't ready yet, but maybe some day."
Your friend is thinking about this way too much. She's 4!!