Sorry. This child is your kid. If you love his father and want to be with him, you need a major attitude transplant. It’s not the little boy’s fault that his mother is a loser or that he hasn’t been given the love and attention he needs. He is only 4. There is time to turn him around to be the loving boy he was meant to be. But not if you add another rejection to his store of rejections.
He tried to kick you because he was feeling threatened. He knows you don’t love him. He knows that the new baby will get all the love and attention he is longing for. He was expressing his anger at the spot he is in in the only way a little kid can — by acting out his feelings instead of talking them out.
I admire your fiancé greatly. He has not abandoned this child. He works hard and has stuck with you in spite of your attitude. You’d be foolish to leave him because you aren’t mature enough to love his son. If you want to keep the father, you need to get some counseling and learn how to be a kind and caring mother to a little boy who has been given a very bad deal in life already.
The added bonus is that learning how to care for him will make you a better mother for your biological child as well.
I wish you well.