JMHO, yes, 4 lbs is too small to have puppies.
For me, the risk of loosing my sweet girl would not be worth the gain of a puppy or two.
Gracie Ruth is 4 lbs and there was never a question I would have her spayed. People ask me all the time if I'm going to breed her. I tell them absolutely not. She's entirely too small, she's my baby and I wouldn't knowingly put her in danger. She trusts me to keep her safe and that is my choice for her.