K.U.
Eh, she's being a girl. My daughter is 4 and can be the same way sometimes. They really can't understand everything that goes into what we do to take care of them or to give them a special birthday party, and they tend to live in the moment, so probably all she could think about was how she wasn't getting her way at that time. And maybe it's all the excitement and anticipation about the party and being overwhelmed. My daughter can be quite the drama queen and it can be 10 times worse when she is hungry or over-tired. But I think you did fine, because I probably would have reacted the same way. It's like, seriously?
I think it's okay to "lose it" sometimes, within reason. Not screaming or empty threats or crazy stuff like that, but having them realize that the world does not revolve around them, their parents have feelings too and are not super-human, and there are limits to what we can tolerate. They need to start learning some perspective, and be thankful for what they do have, rather than thinking it's the end of the world because things are not always going to go their way. My daughter is not a brat, but she does have her "bratty moments". She decides she is not going to cooperate, listen to me, whatever. Then I remind her that we can make this easy, and life will be sweet and fun, or we can make this hard and life will not be so fun - it's her choice. That usually gets her to straighten out real quick. And sometimes I just gently remind her that it's okay to be sad about something, and then at least she feels heard.