Even though 4 week old pups can eat some food, we prefer to leave the mom in longer. There are many different thoughts on this, and I know we used to send pups home at 5-6 weeks of age thru the 70's 80's, & 90s before society deemed it "bad". I don't disagree with leaving in a little longer because it was hard for new owners who had no experience with babies, but remember not to let people who assign human emotions to animals dictate what is natural for the animal. In the wild, moms start weaning at 4 weeks because little teeth hurt and mom's will eventually nip at the pups and can hurt them.
I never offer mash. Instead get a tiny bite hard food. Don't wet it. Don't give them extra milk. Basically the same thing that mom should have been eating all along. Just be sure to use a pan the pups can eat out of. Wet food can sour, and even it doesn't make the pup sick it will attract pests.
Pups can start drinking at this point. Be sure that the water bucket is something the pups can drink from, but not drown in. Also, water should always be the first thing in the front of the pen so you can see inside of the container at all times.
When pups turn 4 weeks old, most moms will start to loose some hair. I can be a patch, or they can go almost completely bald. As long as there are no sores/red spots, I would not do a thing. This is going to be more noticeable probably on a smooth coated dog, but it will happen with most breeds/coat types.
This is know as "blowing their coat".
Smaller litters that get overweight tend to have this issue more than others. Could also be something you see more in other breeds, but just want to make sure that you give the pup time to correct themselves.
However, if you cannot bend the pups leg into position for him, then that pup does need to go to the vet.
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