You mentioned 4 children, and one of whom you referred to as a "step-daughter" and that your wife was pregnant with this child at the time you married her. Seems a little puzzling to me, if you are the "only father" this child has ever known, that she would be referred to as a step-daughter.
Perhaps neither here nor there, but it does make your relationship seem like it might have had issues from the beginning. How long were you married? You mentioned things were good for 6 years or so - - is that about how long you've been married?
Good luck. From what you've said, I don't think you owe her anything. I hope you both can have a mature attitude about this for your children's sakes.