Could your eldest daughter be craving your attention? Sometimes negative attention is just as rewarding to some children as positive attention.
She has maybe by accident hit upon an action that get a huge reaction from you and lots of attention. So she keeps on repeating it.
Before it happens again. I would explain quietly and Undramatically that her actions are very wrong and if she does it again she will have to sit alone at the kitchen table, or on the stairs whilst you snd your younger daughter have time alone together in a different part of the room.
Explain that if she upsets her younger sister in this way that you will have to spend time alone with her younger sister. To help her get over the upset that she has caused.
On the other hand explain if the bad behaviour is not repeated then you will have this time to spend with your older daughter doing something which is age appropriate to her but not for the younger child. Eg cutting out and dressing paper dolls, or making repeated patterns with felt tips eg three red blobs, one blue, two yellow, three red what’s next? Something a 4 year old might struggle with but which a six year old would enjoy.
So perhaps the younger child watches her favourite tv programme whilst you and your eldest daughter do an activity together.
The younger child must not feel left out. It will take careful managing. Do you have another adult who could help entertain the younger child for a short time or sit with her whilst she watches the TV?
Aim to ignore bad behaviour, while keeping the youngest child safe, but reward good behaviour with lots of attention.
I don’t know what the background to this behaviour is. My approach would only work if the issues involved are caused by your eldest child craving your attention.
If the behaviour continues even though she gets lots of positive attention from you then you must seek specialist help.
It is a very trying situation for you.
I do hope that you are able to resolve the situation.