New to this, how do you count weeks?

Ka

On Wednesday my doctor said "you will be 4 weeks tomorrow." Does that mean I am 4 full weeks or starting day 1 of week 4? So 21 vs. 28 weeks?

So I was 4 weeks Feb 28th and my ultrasound is March 13th. I thought she said I would have a 5 weeks ultrasound, isn't that 6 weeks?