It was good of you to take her in four years ago. Poor kitty, changing homes is so hard on them.

I think you should put a litter box inside the house for her. Since she is not asking for the door to be open any more, she needs a box where she can get to it when she needs it. It may be cold for her out there, also.

She may be sick, or in pain, and that is what is causing her to be aggressive to the younger cat. Pain or illness will make her feel vulnerable so she will warn the other cat away.

Keep in mind that cats are masters at hiding illness, so when they begin to show symptoms, whatever is wrong has been going on for a while and is probably serious.