Hi and welcome to BYC. So sorry you are losing your birds. I have a few questions, in order to try and help.
You said you have 8 grown birds in a 4x6 coop but you didn't mention any outside run that they have access to or whether or not they free range during the day. Also knowing what you are feeding them may help to answer your question.
For starters, if there is no run and 8 grown birds have been housed in a 4x6 coop they are too confined and living in their own droppings. Hopefully this is not the case and there is a run but wanted to mention it anyway.
Second, they need extra feed like scratch and leftovers, especially if sick and confined to small areas. Try scrambled eggs.
Third, I agree, they need worming - try Wazine in their water.
Please try and provide some more information so that we can help you before you lose any more birds. It should not be the temps killing them. There are many threads on here where the chickens are running around in negative temps.