It's easier to appreciate how your younger DCs are when the eldest is being a pain. DS is worthy of your protection, and his feelings count. I don't think it will help if you stop talking to DD. Ignore negative, attention seeking behaviour (other than physical violence which should be addressed) without ignoring her. If you show emotion, she'll feed on it and the problem worsens. She's testing you. You don't mention a DP, do you mind me asking is this behaviour reserved for you, how is she with her dad?
If she and the younger DCs show temper and squabble with each other, put the responsibility on them. For every fight they all get consequences no matter who starts what. Domestic chores are a great deterrent so start giving out jobs, eg vacuuming or sweeping the floor, sorting out socks, emptying the dishwasher, cleaning bikes etc. It's important to take care of each other and while it is all right to be angry with each other, it's not all right to hit or be verbally abusive towards one another. Eventually they'll prefer to work something out, before you step in.
Is DD happy with herself? What's going on in her social group, how are things at school? Does she get privacy at home, is she fighting for your attention? When she talks to you, when possible give her your full attention. What if she kept a journal or diary, somewhere she can express herself. It's private, so you won't read it unless she wants you to see a specific chapter.
Hold a weekly family meeting where everyone gets a turn to talk, one at a time. Talk about what works and what doesn't for the whole family. It needn't be for more than half an hour, but everyone has a voice. As a parent you aren't taking sides, but ask each DC calmly, what they think happened that week, and encourage each one say why their feelings are hurt or in DD's case, why she was angry. If they can talk without calling each other names, and work things out, you should see an improvement.
Each DC has an important role in the family and is important in their own way. So comment on specific contributions, make each one feel valuable. And tell DD and your other DCs you love and appreciate them and do it often. After all, with four DCs you can't show favouritism - you love them uniquely.
Ask DD to draw up a kindness chart, a point system, and prizes. Being kind can be a habit. She's become used to being mean to DS and now she's extending it to you too. Maybe you can teach her to stop and consider just before she lashes out. Rewards can be 1:1 time with a parent, extra time on a pc, choose a pudding. The trick is, they all get a prize.
Whenever anyone on the family spots someone else doing something kind or helpful, add that person's name to the chart, parent or child or visitor.
Explain the idea to the rest of the family. Let them come up with ideas maybe include stuff like:-
sharing without being asked
talking not shouting
helping to lay the table
helping a DB or DSis with a chore eg put on their shoes
finding a sibling's mislaid book/toy/article of clothing
reading a story to/playing a game with one or more DB or DSis
putting away toys, etc.
Maybe it will help everyone to stop, think, and be nice.