how to change newborns name he's four daus old in california?

if his birth certificate has been processed it matters not one iota whether he's four days old or forty years old, the process remains the same, which is to officially change it through whatever agency handles it depending on where you live. If the birth certificate has not yet been filled out and sent in or handed in then simply ask for a new copy of the form and start it over again.

If he isn't out the hospital you can do it there I did that personally it's easy just have your partner with you if you named the baby with you there on paper good luck:)