First off, my guess is that she is not doing this because of the past incidences and his emotional withdrawl, but because she is a growing girl and going through a phase.
You haven't given as much information as I would like, but here's one thing to consider: up until things resolved between you and your husband, your daughter likely had *you* all to herself. And then things got better between you and Daddy, and then she had to "share" Mommy. Now, at an age that is typically more bossy and negative (fours are like this, at least from my experience, four year old girls can be a lot of fun and also amazingly mean, without seeming to understand the hurtfulness of what they are saying/doing) and asserting more of their personality, she also has a baby brother to contend with, who is only relatively recently mobile and that in itself is a big adjustment for older siblings. That's a lot.
One thing you can do is to also take a look at what her role models are/do. What/who is she watching? Are the main characters catty and demeaning, because I see LOTS of girl characters like this, even on 'educational' kids shows. So, sometimes kids also emulate what they've seen. This may be one culprit/added negative influence.
I'll suggest two books: "How to Talk So Kids Will Listen and How to Listen so Kids will Talk" by Faber and Mazlish. This book has the potential for improving honest communication and problem solving. The second is "Taking Charge: Loving Discipline that Works at Home and at School" by Joanne Nordling. I like this book because it really uses observation to separate self-indulgent behaviors from bona fide disciplinary issues, and helps parents to reinforce their child's self esteem in the most profound ways.
It also pops into my mind to ask "What exactly happens when your daughter is mean/rude to Daddy?" Do you sail in to correct her, or does your husband stand up for himself? If YOU are doing the correcting, instead of him, that can also undermine his relationship with her. It's okay for him to say "You know, that was a hurtful thing to say, and I don't like it. You may be friendly with all of us, or you can go into your room until you feel you can be friendly again." This is an oversimplification, really, however he needs to be letting her know that it is unkind and he doesn't want her to talk to him this way.
Family counseling is also another option, if you and your husband are willing. If you feel this is an anger issue on her part, having a counselor present can facilitate her releasing her emotions and finding honesty in her relationship with her father.
I hope there is something here that gives you help and hope. It sounds like you and your husband are working in the right direction of coming together as parents. Kids are such a challenge, so try to view this as an opportunity for growth (albeit a painful one, sorry!) and keep on striving for wholeness as a family.
Best wishes,
H.