I'm not so sure you'll get many responses at this hour, but I hope you will!
4 weeks is very young---too young to be without the mommy actually, unless the mommy was killed or something. Hopefully one of the more experienced people will respond to you soon and help you out...but much care is involved in raising a kitten that young.
What are you feeding the kitten that young? How old are the others? Make sure that the kitten is kept warm and safe from other kitties, so they do not accidentally (or on purpose) hurt her.
The kitten should be fed a milk replacement. You can find this at walmart, or a petstore for sure. DO NOT feed the kitten the cow's milk that we drink, as they can not digest it. You could also feed the kitten wet kitten food, but make sure it is KITTEN food, because adult food doesn't have the nutrients it needs.
I can't think of much else right now cause i'm so tired. Good luck with everything, I'll post some helpful sites for you tomorrow, but hopefully someone else will get to it before me cause it's important for you to read up and educate yourself on raising kittens.