Help! My 20 Month Old Girl Is Becoming Violent

Four kids under five is tough, for anyone! You are a wonder woman!
One thing I have learned by trial and error is kids need to feel validation in their feelings. It is okay for her to be angry or unhappy but teaching her what to do with those emotions. She doesn't have to like to share, but she needs to or the toy is not going to get played with! She needs to learn if she cannot play nicely she is to be removed from the area.

She probably does seek attention, however she needs to figure out acting out and being agressive isn't the way to get it.
Put her time out over and over but walk away. She is getting attention even if negative if you hold her, even if it takes a million times, don't let up, be calm and if she kicks and screams just keep picking her up and setting her down, being firm and don't break. Walk away from her. If she reaches for her sisters hair, grab her hand, don't squeeze hard but look her straight in the face, tell her "NO WE DO NOT BE MEAN" and set her down immediately!! I hate to say it but ignoring her and walking away during bad behavior and only rewarding the good is the way to get it through to her. If after time out she continues to hold a grudge, put her back, even if this process takes an hour or two, it is worth it in the long run. Make sure she is well rested, sleep deprivation is a killer on kids behavior too.

She is young, confused and frustrated she cannot communicate with you on what she needs. Talk to her, say "I will give you mommy time when you calm down" or tell her if she can talk to you about what she needs you will listen and walk off. Give her words and be compassionate to her frustration, but she needs to figure out that life is not fair and she isn't going to get her way. Sharing is hard for any child, with three other siblings in her stuff, it can be really hard.

Get boxes, mark each childs name on the boxes. Those are the "SPECIAL" things only for that child. Like a bear, a favorite book or whatever. Everything else is fair game. I babysit and always tell my kids I watch and my own, if you put it down it is anyone's to play with. They have their own special things, blankie, a stuffed toy they do not have to share but it has to be put up too and they are responsible for it.
If she has some things she can keep to herself it may lessen the stress of feeling like things are being taken away from her.

Comfort her and love on her when she is sharing, really praise to the hilt when you catch the good behavior, like THANK YOU SO MUCH FOR BEING SO SWEET!!! When she acts up, tell her it is not okay and she needs to go cool off and set her in time out. Don't hold her but continue it, it is exhausting and I am sure not something you want to deal with seeing you have three other children but if you stay on top of her, consistent punishment for the crime and DO NOT GIVE in, then she will get it!
She knows your breaking point.

She has two other younger siblings that will soon mimic her behavior. You have your hands full too. Maybe just you getting out once or twice a week, by yourself and helping you relax will help her too. You need to take care of yourself! :)

It isn't easy, for just my two it wasn't and I just really set it up that I have zero tolerance for not sharing and being agressive!!!!! Some may not agree but I got a punching clown when my daughter showed some aggression issues, I told her if she was so angry she couldn't talk to me to go punch the clown or her pillow and get it out!! BUT IT WAS NEVER OKAY TO TOUCH ANYONE ELSE!!!!!!! It helped, believe it or not it helped. Giving her tools to get out the anger and then talking to her about her behavior will eventually help.
Empowering with being a good big sister and a good example helped my daughter feel she had a purpose too!

Good luck, no magic answers and I am not an expert by any means. I just know with punishment and time outs my kids know I will not cave and it will make life worse, to serve their time and then we can talk about what happened!!

She needs more dicipline. When she is acting out get her in a full body hold and hold her until she is calm. Tell her there is no hitting, or screaming or throwing. It may take a few days. Also, I would read the book, 1, 2, 3, Magic. It's wonderful. You may want to try it.
When she hits you put her right back into time out. A couple of days of not being able to play might help her understand that she's better off getting along. This is part of the terrible two's and horrible three's. Good luck!!!

You should watch some episodes of Supernanny. I always get good ideas from her show. There are also some way out of control kids on her show. It might help. I know it has helped me.

I see you have had a lot of responses and maybe some good book suggestions, but it seems many of the responses are basically things you have tried. This sounds like a job for Super Pediatrician! If you have one you like, you should be able to get some good advice and support. Good luck.

the only thing that works with my dd (almost 3 now) is to tell her it is okay to be upset and it is okay to cry or scream if she needs to, that her bed is a safe place for her to get her cries out and then please come talk to me I would love to snuggle when you are ready to.
I wasn't sure how well it was working, I mean we are consistent with it and it helps but this is such a hard age for these little people--trying to figure out how to deal with all these feelings--learning to talk, to be big people instead of babies--anyway we were driving home late the other night and she was so whiney! I was getting frustrated and had just had it and I said "Mommy can't take it any more tonight I need you to stop" and she said in her wavering teary voice "I just need to get my cries out first"
it put me in my place I'll tell you what...and it helped me to see that we have been doing a good job giving her a safe outlet to get emotions out.
when she hits she goes to time out, she can't take anything with her, I usually have her sit on the stairs
If she starts crying she goes up to her bed to get her cries out and then has a time out. I usually count to 20 or 25 for her at this age now. then she comes and talks to me and I have her put both of her hands on my cheeks and look me in the eyes while we talk so that I know we are communicating and it is getting through
well now when she has something to talk to with me that is serious to her 3 year old self, she comes and takes my head in both her hands and says "look at me mommy"
I don't have other children around every day, so I know that makes it more challenging (I am the 3rd of 13 kids myself) good luck--