If you are 4 months pregnant and the baby heart rate is low what do the doctor do to raise the heart rate to normal?

There is nothing: that can be done this early in pregnancy. The baby must be at least 23 weeks to be able to survive outside the uterus. All that can be done at this point is to watch it. Close follow up with your OB is obviously very important. That being said, a low heart rate is not necessarily a major problem and is often temporary. Hope this helps!