A: It sounds like the 4 year old has been reinforced for the negative behavior at some point - otherwise she wouldn't do it and expect it to get her what she wants. She may also be jealous of the younger one. Just by nature of being two, they will demand more of Mommy's attention, and that is hard on older siblings.
My books have a ton of strategies (Toddler ABC Guide is a quick read, Life with Toddlers much longer), but overall, you have to start changing your thinking. Look at WHY she's acting that way and make the consequence the opposite of what she's trying to get. More importantly, you have to start guiding her on what you want to see instead. She is trying to get your attention, or get her way, etc. and she doesn't know another way to go about it until you SHOW her. Tell her and show her how to ask and act appropriately to get what she wants. The behavior has to stop - absolutely. But if you aren't getting through to her, you have got to try a different approach to your consequence or "punishment". You have to make sure you are not unknowingly reinforcing what you don't want to see. Yelling at her for the behavior is actually a reinforcer if the goal is attention...see what I mean? And if your attention is often tied to the 2 year old, she'll try to get him into trouble so that you'll "see" her.
I know it is SO difficult with a toddler in tow, but you also need to make sure that the older child is getting enough positive attention. This negative behavior is generally a result of wanting mom's attention, and not knowing how to act and what to do instead to get it. Make sure that your interactions with her are not turning into exasperation most of the time. She will sense it and get more and more out of control. You have to step back, look at the big picture of what she is trying to gain with the behavior, and address it objectively. That is nearly impossible (Life with Toddlers is ALL about it), but it has to be the goal if you want to change the behavior. It starts and stops with the caregiver.
So. Step back, be objective, change your approach to discipline/consequences/punishment, and make sure she is getting positive attention for the good that she does.
You can do it! Hang in there!