How Babies Are Made is an excellent book! I forget the author's name, but I believe it is still in print. It has charming illustrations that nonetheless give a clear idea of what happens inside (cutaway views). It talks about how plants make seedlings, how chickens make chicks, how dogs make puppies, and finally how people make babies. This is the book my parents found when I started asking questions (and I was 2...so 4 is not too young!) and I wanted it read to me again and again.
Not only was I fascinated by the subject, but that book just makes it all seem so happy and natural.
Mcaws, for your 9yo I think a book is the best approach. If you still read to him at all, then read it together, but if you don't then he would find that weird, so tell him you'd like him to read the book so you can talk about it. Then, talk--not just "Do you have any questions?" "No." "Okay."--talk about people you know who are having babies, about how IVF is different from the old-fashioned way and why you did IVF (don't underestimate his ability to understand things like blocked tubes, but back off if he seems squeamish), about when and how you learned where babies come from and how you felt about it, etc.
Please don't "leave it to DH". The people I know who have the best attitudes toward sex are those who got some information from adults of both sexes. When mom talks to girls and dad talks to boys, it can give the impression that sexual topics mustn't be discussed in mixed company--which creates a problem in communicating with future partners, unless you are gay.
Oh! You didn't say how old your younger kids are, but maybe you could ask 9yo to read the book to a younger sibling. Then you can educate 2 at once, plus help 9yo feel like he's teaching something which he may feel he should already know (there was lots of discussion among my peers by 9!), plus give another tactic for asking questions ("Little brother wants to know when he will have sperm.").
Good luck to both of you!