Pregnant with 4th, questions about placenta, c-sections?

First of all, let me preface this by saying that I am a medical student, so I might have a little bit of insight, but I am far from being a full fledged doctor. From what I understand of the matter, a retained placenta due to atony of the uterus would not be related to placenta accreta. There is a much higher risk of you developing placenta accreta because of your history of c-section births, and I would definitely bring it up with your doctor, but I would be more worried about a recurrence of a retained placenta than placenta accreta occurring with your current pregnancy. Good luck, and make sure you are open with your doctor and I'm sure everything will be fine.