When you say she's stopped nursing the pups, is she behaving oddly in any way, hiding away for example? If so, there's a possibility of eclampsia in nursing bitches which needs urgent veterinary attention. But if she's fine in herself and is just not wanting to feed the pups, have you trimmed the puppies' nails yet? By 4 weeks old, puppy nails can be v long and v sharp which will be painful for Mum, making her reluctant to want to feed her babies.
