Lethargic 4 week old kitten, vomiting and shaky?

I have a litter of 5 kittens, they were all very healthy and lively and they are just now starting to eat regular food and drinking water along with their feedings from mama cat. However, today one of our kittens was showing signs or lethargy and she just slept all day long. I got worried because she wasn't eating or drinking so I got a syringe and and fed it some water then bought some KMR formula and fed it that as well. I wrapped the kitten up and just let it be hoping it would improve, but tonight the kitten is still the same only more shaky and it actually vomited up the formula I fed it. I can't afford to take her to the vet unfortunately, but i'm desperate to help my poor little kitten. any answers are helpful! :( all the other kittens are just fine.