You might want to get her checked out just to be sure since plenty can go wrong but it is normal for a nursing animal to lose weight and the more babies the worse it is. It takes A LOT of work to keep a nursing mom looking even somewhat normal. Have you tried some canned food? You can also ask your vet about A/D which is awesome. I always have a case around and use it for moms and weaning. I probably wouldn't pull them quite yet unless the vet says it has to be done. Professional breeders keep their kittens minimum 10 weeks to 16 weeks for good reasons :-). Otherwise mom and kittens seem good? No fussing or crying or dying indicating a problem?
If they are 4 weeks you can probably start weaning now and make it easier for mom to keep some of her nutrition for herself. Start putting down some canned food in a shallow saucer that's more watered down at first (not too cold or hot) since they are used to a liquid diet and increase the consistency as needed based on them. You can do this a couple times a day at first and you'll probably have to move mom so she doesn't eat it. They'll start eating the dry food soon too. They can probably go around 8 weeks if you have to.
If they are 4 weeks you can probably start weaning now and make it easier for mom to keep some of her nutrition for herself. Start putting down some canned food in a shallow saucer that's more watered down at first (not too cold or hot) since they are used to a liquid diet and increase the consistency as needed based on them. You can do this a couple times a day at first and you'll probably have to move mom so she doesn't eat it. They'll start eating the dry food soon too. They can probably go around 8 weeks if you have to.
