4 year old very sassy

Sassiness and wanting to do things her own way is very normal. Having fits is very normal. Trying to self harm is not normal. Sounds like she’s dealing with a lack of control which is normal. Try to help her feel like she can regain some of that control. Let her pick snacks, meals if possible, clothing, shoes, activities. One thing I started doing when my daughter was 3 was when she was refusing to do something like put her clothes on I would say either you do it or Mom does it. She would almost always choose to do it herself. With her outbursts I recommend deep pressure, hugs, gently squeezing arms and legs. Laying on her with a little bit of your weight. Having her push or pull heavy things. Sometimes she pushes on my hands with her hands or feet. I have my daughter do these things before bed but they are also great to practice during the day and using them during melt downs.