Sick 4 week kitten!?

My kitten was fine up until about a week ago. She was eating fine and now all of a sudden she got very skinny, I am trying to bottle feed her but she is too weak and wont even try. The mother cat WONT feed her or even bother to try. I dont know what else to do to try and save her. She is also the runt of the littler. The other 3 are healthy little fur running around getting into big messes, but this one is always left out. Is there anything else I can try or is it to late for her?