Here's another thing I'd also like to mention...seeing that this Momma is missing teeth (tongue hanging out) gives me a good indication that she may not have always received proper care & nutrition.
Do you know if your Vet is familiar with toy breeds? Believe it or not, not all Vets are and she could be at a disadvantage if she does encounter problems with her pregnancy or during whelping and you could end up losing Momma & the litter as well.
Also, if she weighs 4lbs now...she was even smaller before pregnancy.
Maybe Mardelin could weigh in here and give you her thoughts on this situation. I just want to make sure you fully realize the gravity of this situation.