N.T.
To me it sounds like Abby is needing extra attention. I have found in my 15+ years experience with children, that sometimes a well mannered and well behaved child can change overnight if something is upset in their world, something that is very small and insignificant can cause waves of disturbances. Their behavior can be a way of showing that something is going on because they have complete control of how they interact with others, and if they are out of control of something, say moving to a different room, they retake control and it appears that they are misbehaving. The birth of your son created a huge shockwave for her, and since some of the other ways she has probably tried to regain some of the control/attention haven't worked, she's being defiant. That gets her negative attention, but at this stage she is desparate and is acting out because she knows that without a doubt she will get attention...it doesn't matter how she gets it! She needs more hugs, more kisses, more love words, and some special time with her mommy and daddy away from Dillon. She needs to feel special in her own world, and I bet if she gets that, you may see a change very quickly. I know you have probably heard before to catch her doing good things, and praise her for doing them, and it sounds like a quick cliche, but it takes practice and a consistent effort in doing it.
I would stay away from rewarding her for behaving well, because she should behave well and feel good about it all by itself. You would be setting her up for a reward system for her life, and if she didn't get a reward, she would not be motivated to do whatever the task set before her for years to come. The things we do now are the foundation for the way they live their life later. Stick to the counting to 3, then give her consistent discipline EVERY TIME she does not do what you ask. Also, be aware of how realistic your instructions are, and if they are followable (if that is even a word) by a 4 year old. I often tell my husband and my other mom friends to be aware of what you ask of your child, and to choose your battles well. Sometimes things can slide if they are age typical misbehaviors that can be nipped at the bud by immediately sitting down with your child and explaining in terms they can understand why that is wrong for them to do. Use words they can understand, and like I said before, be consistent. If you discipline for something once, you must keep disciplining over and over again. Consistency is key in raising children, which you probably already know.
So in addition to counting to 3, Abby must know what the consequences of her misbehavior are going to be. Also, let her know that you love her no matter what, but she will have to pay the consequences for her misbehavior. Spend more one-on-one time with her whenever possible...and I don't mean like huge day activities, but 15 minutes reading her favorite book or sitting and having a special tea party with her and her alone with all the trimmings include little sandwiches and maybe her favorite drink, not just tea. Make her life special which will in turn make her feel special and loved and secure, and will give her the attention that may thwart misbehavior.
I hope you don't think any of this is talking down to you because I didn't intend any of this to do it. I do all that I just encouraged you to do, and tell my friends and family the same thing when they seek my advice. Good luck, and I hope you keep me posted on your progress.
Good luck,
N.