Okay first things first, calm down.
I can imagine how horrible this is for you, but she is not abusive, she is FOUR. A four year old child. I don't know if maybe having older children is causing you to expect more from her and put more mature motivations onto her behaviour, but she is only 4. She is not trying to abuse you, she probably barely understands that she is upsetting you. She hasn't got the emotional sophistication for that. So for starters try and take the emotion out of it a bit; it's perfectly natural to be upset, hurt, and anxious; but don't demonise her or position yourself as a passive victim with no power. You are the adult; you have ALL the power.
Are you saying your other 3 children never acted up? Never went through a hitting or a biting phase? Never pushed your boundaries? If so, you have been astonishingly lucky thus far.
How are her communication skills? Has she got any delays? is SEN suspected? This is important context for understanding the behaviour.
What are her triggers? Or does she just wake up spoiling for a fight?
Is this behaviour new or has she always been reactive/violent/difficult to control? This would bring me back to the question re SEN and communication skills.
In terms of managing the behaviour, at this point you have to protect yourself - if that means putting her in a different, safe room away from you or even shutting yourself in a different room (leaving her somewhere safe and secure), or gently restraining her so she can't hurt you or herself if not at home, then do it. Throughout this you should be calm, and verbalise to her "I can see you are feeling angry/frustrated/upset, but I will not let you hurt me." Suggest other ways she could get her feelings out (punch a cushion for e.g.) and offer her comfort (in whatever form will work for her, for some kids it's a hug, for others a hand-hold, or to talk about their feelings) when she is ready to receive it. If she repeats or escalates the violent behaviour, return her to the safe space or re-restrain and repeat until the storm passes.
Unless she is a particularly large or strong 4 yo or you are particularly petite, you should still have the physical advantage and be able to overpower her without hurting her. If you struggle to move her to a safe space or to restrain her from hurting you, do you have any back-up - another adult like a partner or one of her older siblings - who can support you?
God idea re HV. But I think the most important thing to do right now is check your attitude towards her - she is not abusing you, you are not her victim, she is 4 years old and her behaviour is communication. If you take this victim role, she will feel even more scared and out of control and her behaviour will escalate. You need to be her calm place in the storm. She needs to know exactly what you will do in response to the behaviour - and what you won't do. She needs calm, loving, rock hard boundaries. You can't give her those if you are in a victim mindset.