Canned cat food is a major culprit in causing diarrhea in kittens due to the high amount of carbs in it. A four week old kitten usually does not begin to wean onto solid food yet, they usually only drink their mom’s milk. I would hold off on the canned food and only give her the formula & see if that helps. Kittens typically do not begin solid food until around six weeks of age, and at six weeks I’ve always given my kittens a little bit of soft food three times a day along with their formula throughout the day on demand.
If you really, really want her to have solid canned food at this age, try a canned cat food with a higher protein content, like Blue Buffalo, cause they usually do not cause diarrhea in kittens (or at least that’s been my experience). Diluting the formula usually only helps with constipation. If only giving her formula does not help with her diarrhea, then try giving her some mashed up rice. Rice usually helps both firm up stools and calm the stomach. I’ve heard other members recommend fresh pumpkin, but I have never tried that. The rice usually helps for me.