If there is a four day difference between my lmp and ultrasound date, don't i go by ultrasound and could that mean i ovulate late?

Within 4 days is : No real difference. There is nothing magical about either date, except for 2 things: 1. If the LMP is different from the early OB us dating by more than 2 weeks, the LMP is probably wrong. 2. What everyone wants to happen is appropriate interval growth - meaning the fetus grows in size to match the time since the last us - that means growth is happening normally.