To me it sounds like she's a stray and is just shy of people at this time. If you live in a small apartment, this might be kind of difficult, especially since you already have your four kitties there, but if you trap her, maybe you could bring her home temporarily, just to reacclimate her to having people around. Then when she's ready, you could take her to your shelter to find her forever home. Seems like she would get used to people again, but would need to be trapped and forced into it.

I hope others have better suggestions, but that's all I could think of right now. I wish you luck in whatever course of action you take.