Why are my 4-week-old puppies dying after starting wet food?

If they are nursing standing up they may be throwing up because of that. If mom doesn't want to feed them anymore, I suggest you start transitioning them to solid foods. The best one would be this one from Royal Canin: https://www.chewy.com/royal-canin-starter-mousse-mother/dp/118070

And in terms of the food -- he should be willing to eat wet food, if not dry food, at this age. So if he's not, I would highly recommend a check up with your vet to make sure everything looks ok. He may just need more time to adjust, but with such a young guy, he can get low blood sugar and dehydration very quickly, so it's important to act sooner rather than later.

At four weeks of age, you should be able to start trying to mix in some solid food, or potentially a gruel with wet and dry food together. Dogs will start getting their first puppy teeth right around the age or 3.5 to 4 weeks so they shouldn't have too many problems with the mechanical digestion of the food. If you feel like the puppies aren't eating enough regardless and are failing to thrive, I would get them in your vet for a visit. Hope this helps.

It's possible the change in food could be causing digestive upset. The best way to start transitioning to solids is to make a slurry of their regular milk replacer that they've already been on combined with a little bit of the puppy food. This helps make it easier to digest without being too big of a change and may help settle their stomachs. Checking for any parasites with a fecal exam and deworming may also help, however if the vomiting or diarrhea continues, a vet checkup to rule out anything more serious is best.

Purina puppy food is fine to feed. However, I would also deworm the puppies with a puppy wormer available from your veterinarian or pet shop. If the diarrhea does not stop within the next few days, I suggest you take the litter to a veterinarian for a puppy exam and fecal exam.

Ideally, they should stay with the mother and litter until at least 8 weeks of age. This can prevent behavioral issues as they get older. They can start weaning to solid foods beginning at about 4 weeks of age. If they can stay with the mother and litter while weaning, this would be best for them in the long run.

Puppies get weaned between 4 and 6 weeks, so it is probably just time! I recommend mixing water with canned puppy food (or dry puppy food) to make it really wet and mushy. The puppies will often play in it and then lick it off their paws. Here is a helpful article: http://www.peteducation.com/article.cfm?c=2+2108&aid=887