As stated above, a 4lb girl is definitely too small to breed. I would suggest you take your vet's advice and have your girl spayed. Your baby will not be any less lovable if she does not have a litter.
To be a repsonsible breeder, you need to do extensive research and find a girl who is a quality representation of the breed standard and worthy of producing offspring. There is much more to breeding than mating two dogs. There are many many things that can go wrong, including mom and pups.