I've had 4 ultra sounds all giving a due date the same or with in a week of each other. I'm still worried they could all be off by 4-5 weeks. Is this possible?

Not likely at all... its important to know that ultrasounds are never precisely accurate regarding gestational age and are usually off by a few days but there's not a chance it would be off by 4 to 5 weeks as the baby hits certain critical stages the further it progresses and the both the physician and tech will be able to tell that.. if it were off by that much they would have adjusted your due date

Ok. My due date was September 29th going with my lmp. But my last doc visit he moved it to September 23rd and said the ultra sound was more accurate. I'm embarrassed to ask him window of conception, is it possible to know this?

Its almost impossible to know exactly when you conceived as you would have to know when you were ovulating and that varies from woman to woman.. you can ask your doctor and he may be able to give you a gestimate of when you conceived... it sounds like you might be worried about 2 possible fathers... just be honest with your doctor... they aren't going to judge you.. just be honest with you.. I know it seems embarrassing but the only way to know is to ask the question... you can get advice and opinions here but none of us are qualified physicians therefore our advice is only that and what we have acquired by experience... ask him... trust me its not that bad and you may get some peace of mind