She's 4, and this is what happens.
MOSTLY a child needs to learn how to express themselves, good or bad, in a palatable way... so they don't feel censored and can tell you how they feel.
teach her the names of feelings etc.
Validate a feeling/mood... then give her ideas about "problem-solving" that... with your help. A child this age is not equipped to emotionally manage their emotions much less understand them.
So teach her. I tell my kids, its okay to feel frustrated/grumpy... but we are a "team" and I will help them address it and to "try their best" to tell me what is up etc.
Also, the book "have a new kid by Friday" by Leman is great. Amazon h as it as well as E-bay. It is not punitive and very understanding.
The thing is, you want a child to be "emotionally" aware... otherwise they grow up being an older child or adult that cannot express themselves, nor know how to cope, nor know how to manage their feelings good or bad.
So it is a process of teaching them that and helping them. Not just scolding for any little thing. Like a rock collecting moss... they will over time become articulate about their emotions and know themselves.
My son, is only 3.5 years old.. and he will on his own tell me "I'm grumpy... I want to be alone... I'm going over there, please be quiet." etc. And I am proud of him. He tells us what he needs/what he feels/and manages to "know" that he wants to be left alone. Then he feels better and will come for a hug. That is him. Versus if my daughter is "grumpy" or frustrated, she needs a hug from me and wants me right there with her.
So know your child... and their cues as well.
Sometimes too, explaining and lecturing them in the midst of an upset does not help... they just want to be validated... then after they calm down, talk about it... age appropriately.
Also just talk with her and see what it bothering her. But don't judge her. My daughter needs that and then she feels better.
I don't try and in-validate her... but to see how she is feeling.
Imagine yourself: what if you were stressed/frustrated/grumpy... and you 'vented' to your Husband and all he did was send you to a corner and told you be quiet or that your feelings are not acceptable????? Would that make you feel better or worse???? Probably worse. So, same for a mere child, who does not even know how to manage their feelings automatically. As you see, not even adults know how to do that nor how to comfort the other when they are feeling icky.
A child, has to be able to vent and be themselves at home. Otherwise, they will have no where to vent or have feelings. My Dad ALWAYS said: he'd rather have his kids be able to be themselves at home, warts and all... than have them try and get fulfilled elsewhere outside the home.
If a child is hammered down, like a nail anytime they feel bad or unable to cope... then what? HOW will they learn that they have a safe place to fall...
The Book "Your 4 Year Old" from Amazon is also very good. An easy read about each age juncture in a child.
My kids are 7 & 3.5... they know that we cannot buy everything, due to budget. We taught them that. They understand. We explain it to them in age appropriate ways... about money, about spending, about needs versus wants etc.
Girls can be emotional.
Talk with her....
Kids, do NOT have to 'share' everything. What if your Husband said you had to share everything with your SIL or him? Would you want to? Is that fair? I let my kids... not have to 'share' everything. And especially if something is special to them. Or, you give them time with an object... then transition them and say "in 15 minutes.... its her turn...." If she can't do that, then stop the activity.
Also make sure she naps. Tired kids spin out of control.
Teach her 'coping-skills.'
all the best, just some ideas,
Susan