Welcome to our forum.
Nadya's advice is excellent. I won't have a whole lot more to say about it. When outing yourself, start with those who you can trust to keep it secret with you, until such time as you're ready to have it broadcast across the family. Prepare yourself to answer the questions people will ask by reading lots and lots on this site. The Golden Nuggets board
is a good place to start.
And as Nadya said,
"Then to my opinion, telling a four-year-old is much simpler. You just tell him that the baby has the same father as he does so that makes the baby his sibling."
Hope that helps you get started. Let us know of any other thoughts or questions you may have.