4 week Old Kitten Underweight and really loose almost runny stool

So I got my kitten Mopsy about 3 days ago and when I got her the vet said she was underweight. She was weaned and was on solid food so she said no more KMR but she's underweight and I want to put some weight on her. The vet said she tested negative for worms on her fecal exam. I have her on Science Diet for Kittens wet food mixed with Purina Kitten Chow. She has really loose stool as well and I'm thinking it's because she may have had a sudden change in her diet or because she's still a bit stressed from moving to a new home. She's energetic and loving but I want to make sure she's the gosh darnest healthiest kitten around. I want to keep her from getting dehydrated in case she's losing too much water is there anything I can do?

Most likely the food change is the culprit.  You can try some plain boiled chicken and plain yogurt to help her bowels calm down.  You can also feed her some canned kitten food (Wellness, Fancy Feast, or Royal Canin would be good choices), which should be her primary source of nutrition at this point anyway, and add some baby plain pedialyte to keep her hydrated.  Do you know how to test for dehydration?  Pinch the skin on the back of her neck.  If it stays up like a teepee, then she's dehydrated.  If it snaps back down, then she's fine.  Are her bowel movements frequent?  Did the vet give you anything for the diarrhea? 

I have been feeding her canned kitten food (Science diet was what my vet had recommended) mixed with her Purina kitten Chow and when I came home it seems that it has definitely taken a better shape than what it was. Her bowel movements are maybe every few hours or so. Since I've gotten home from work she has only had to go to the litter box once or twice. I tested her and it snapped back into place and she drinks water frequently. I think a mixture of the stress and the sudden food change that did the trick but now she is looking good. Now all I have to do is work on putting weight on her. Thanks for the advice by the way. As a new cat owner, I'm constantly worried about every little thing.

You can also add a bit of plain canned pumpkin (NOT pumpkin pie filling) or plain yogurt (not low fat or any flavors) to her wet food.  Both of those are good for helping to stop the runs.  In a few weeks, work to gradually change her to a higher grade food, preferably grain free, as it'll be more healthy for her in the long run.  Check out the info  http://catinfo.org/ for more good information on cat nutrition.

I'm going to slowly start changing her food to Blue Wilderness. It's grain free and I read good reviews on it. Thanks for the tips, I've been noticing that it's been looking a lot healthier and not loose anymore! Yay!

Mopsy is beautiful. I love black and white kittens. I have one. He came from the streets. If he's eating wet food and drinking then he shouldn't be dehydrated. My vet told me a little boiled chicken and rice but seems you don't need it now. Isn't it sweet to see cats and dogs playing together. Keep up the good work. More pictures please.