
I don't know if it is "normal" for a bitch to stop eating at around 4 weeks into the pregnancy, but I and a friend of mine have both experienced this. We both breed small breeds, though not the same ones. My breed is one of the bichons.
After much experimenting and coaxing, we both had to finally resort to making the food into a paste by putting it in the blender, and then making small balls out of it, pushing these into her mouth and holding her muzzle closed until she swallowed. (You cannot do anything like this with dry food. You have to soak it.) I know this sounds like something one wouldn't want to do, and indeed, I did not WANT to do this, but I kept it friendly, she understood, and I had to keep on with this for many weeks. She was refusing ALL food, even the most delicious things you can imagine. I couldn't let her go the rest of the pregnancy with nothing to eat! Her poops were perfectly normal throughout, and she digested well.
So even though you may want to still wait and see if she will start to eat on her own again, take heart from the fact that - if worse comes to worse - you CAN feed her, you CAN oblige her to eat, you CAN stave off "starvation"! Naturally, if she were not pregnant, then I wouldn't recommend this last resort procedure, but with puppies on the way, you don't don't have an unlimited amount of time to hold out. And with a small breed, even less.
Of course there is a certain amount of coercion involved, but if you keep it friendly and happy, she won't misinterpret the procedure. And BTW: these females I've mentioned were not ill! It all became normal again, sometime following the birth, but in the weeks leading up to the blessed event, there was nothing else to do!