I think the "right" age to explain things more depends on the child. If your daughter is as intelligent as you say, she may be ready now. Think long term...if you're not giving satisfactory answers, she'll find someone who does and may get information you would rather she not have. At 4, this is an annoyance. At 14, this could result in an unplanned pregnancy. As long as you consistently respond to her questions in a way that satisfies her curiosity, she'll keep coming to you for answers.
Case in point...I have no problem explaining these "facts of life" and my sons (now 12 and 16) asked me questions constantly, particularly my older son. The older he got, the more challenging the questions. I would always answer the question first from a "biology" point of view and then re-answer it from a "values" point of view. By the time he was in jr. high school, I was spewing my soda hearing the questions he was asking, but I answered anyway. Finally, after two weeks of the most outrageous questions, I asked my son where in the world were all these questions coming from!?!? He said that all his friends knew I was the "only mom willing to answer the tough questions," so they'd have my son ask me and have my son report back to them. I was floored.
As for the sharing at preschool concern, I would simply tell your daughter that it's only appropriate for her to have these conversations with you. It's NOT ok to talk about this with other children or adults because it makes them feel uncomfortable. Again, if she's as intelligent as you say, this will make perfect sense to her.
I would encourage you to get the book "Where did I come from?" and it's companion book, "What's happening to me?" The first book is about pregnancy and childbirth and does a great job explaining it for little ones (I was ok with the pics, but not all parents are). The second book deals with puberty, which I know, for you, is a ways off. I had the set my mom shared with me and my sister and when my boys started asking questions, we sat down and read it together. I left them in my older son's room and I know they looked at them and brought me questions. It was a real "learn at your pace" sort of thing and, so far, it's worked well.
Good luck!