Bully being bullied

[Cpointpond]

I have 4 chickens: 2 americanas and 2 rhode island reds that were raised together. The one Americana was a mild bully, but the tide has turned and the 2 rhode island reds have ganged up on her and are destroying her neck and restricting access to food and water to the point I am concerned for her welfare. I currently have injured old bully separated out so she can see the others and have access to food and water. Should I separate them in pairs? I am still putting them all in the same coop at night to keep predators at bay. Any other recommendations?

 

[enola]

How much room do they have?

 

[Ol Grey Mare]

How old are your birds? What gender is each bird? What is the size and setup of your coop and run?