Personally, if it were me, I'd have her spayed.
For many different reasons, but my biggest concern would be for my dog's well being. I would scared to death for Abby or Daisy to have puppies.
Also, take a look at all the rescue sites and see all the dogs that need homes. It's heart breaking to say the least. I did buy my youngest Yorkie from a breeder, but if I EVER get another dog, it will be a rescue or one in need of a home.
I'm not an expert on breeding, but I think the male should be smaller than the female and I agree, 4 lbs is small.
What if she has problems delivering?
I wish you much luck with your decision.
BTW, I have my Abigail scheduled for spaying on Nov. 11. and my Daisy is already spayed.