What’s one more?

So I have 4 children to my ex husband. I am now in a new relationship and my boyfriend is interested in a child of his own. I don’t want another child. I’m afraid my older children will get jealous of the new baby that gets to stay with me full time or the attention it will get. I have a lot of concern going into the potential of having a child with him. I honestly do t want to take it away from him and I know he would be a good father. Does anyone have any similar experience dealing with custody and new relationships.