HELP! I have a 4 week old kitten that started vomiting

My gorgeous Meva had 3 boys which just turned 1 month old on the 7th   Everything has been great, although Mommy ended up becoming sick due to the trauma of birth (it happens sometimes). Her only symptom was loose stool (constant)-the first day she went TWELVE times! I picked up some Ensure, to help keep her hydrated (amazingly water will just cause more diarrhea) and mixed in with her food some cooked white rice to help bind her up. THAT worked VERY well!!! Long story short, the babies ended up sick also-but just head colds.  Sneezing and slight eye discharge were their only symptoms. My dilemma NOW is Sage has thrown up 3 times over the past several hours. They all are still nursing, although Spicy has nibbled on some wet food. Anyway, the vomit is very yellow, and it has white chunks-I assume it's curdled Mommy's milk. I saw a blog or two on the web mentioning said problem, but with no outcome. ANYBODY know why this might be happening? Nothing has changed; Mommy is better, same diet, I watch them very closely, and limit them to just the closet if we go out-which is never for very long. Sage (the one in reference to) WAS starting to drink water from a set of dishes that we bought which I cleaned well, but that was days ago (that we bought and he started drinking) and it's Poland Spring. Never give my babies tap water. Sooo...should I wait to see if it passes? Let him nurse? Or start with the formula?

I thought Ensure was a dairy product, which is bad for cats?

If the kitten is vomiting yellow, it sounds like bile. That means the kitten is pretty sick. If a kitten vomits more that 2-3 times it is time for a vet, in my opinion.

Is the kitten who vomited dehydrated? How is his coloring (gums, ears, paw pads)? The only time I have had kittens that young vomiting bile was when they had Panleuk. Has mom been vaccinated?

I agree with above.  Unless the kitten is apparently fully healthy, its time for an ASAP vet visit.  At least - phone your vet and ask.

You wrote "(amazingly water will just cause more diarrhea"   -  very true.  Its why when animals or people have diarrhea,  some type of  pedialyte is best.

Home made or bought.

Even such product as this Poland water you mentioned, should be better than plain water for these diarrhea stricken.   Lotsa of electrolytes in there.   Just add some glucose sugar, and voila, you will have an excellent home made pedialyte.  Somewhat costly but otherwise nice and good.

Good luck!