My four month old won't sleep unless she sleeps on her stomach what should I do?

This is actually quite normal, a lot of babies prefer to sleep on their stomach, in fact more babies probably sleep on their stomach than those who sleep in other positions. It just seems to be the most comfortable sleeping position for many babies.

That being said, there is a condition called Sudden Infant Death Syndrome where an infant dies in their sleep. The usual culprit is if the baby sleeps with their face in the mattress so they can't breathe but another speculated culprit is a fire resistant chemical found in mattresses, this chemical is normally harmless and not toxic to people, even a 14 month old baby can put as much of their face in mattresses with this chemical as much as they want. However, infants, especially newborns, because they are so undeveloped and their bodies are so fragile, this chemical could be very toxic to them. A method that few parents know to do is to put a mattress protector or at the very least a sheet, something that creates a layer between the mattress and the baby, and obviously they are at greater risk if sleeping on their stomach.

I would definitely say to supervise your sleeping baby as long as you can and try to gently turn your baby over onto their back whenever they roll onto their stomach. If there's one thing to look forward to, the risk is significantly reduced when they turn 13 or 14 months old.