If my lmp was 9th sept but i didn't ovulate til 4 weeks later, will dr still use my lmp or my ultrasound due date?

Depends: Your OB will decide based on certainty of your clinical dates. Not sure how you know when you ovulated. Due date is usually determined based on lmp, because it is generally difficult to know exactly when ovulation occurs. If an early dating ultrasound differs by more than a week or so and/or if clinical dates are uncertain, your OB may choose to use a due date based on the ultrasound instead.