Working out dates

We have recently found out we are pregnant. FF and Glow have updated to say we are pregnant and are saying we are 4 weeks and 1 day. I just want to confirm when do you start counting the weeks in pregnancy? Is it from the 1st day of your last period or the day of ovulation? I'm just confused at how many weeks we are. Also if I was to take a digital test would it show 4 weeks?