I'm guessing you really mean almost 4 weeks gestation.

I'll explain why. Pregnancy is counted from the first day of your last period.

At two weeks you (typically) ovulate. That's right you're two weeks into your pregnancy and haven't even ovulated yet lol

it take about 10 days from ovulation to implantation (some women may notice a small amount of bleeding at this time.

Once the developing embryo (actually it's a blastocyst at that point but hey) has implanted into the lining of the uterus then you begin making hcg (the pregnancy hormone).
First response is the only home preg test that can pick up a pregnancy upto 4 days before your period is due (and with a normal cycle ie 28 days you actually won't be physically pregnant before that)

gestation is counted from conception (approximately the time of ovulation give or take a few hours)

so at 2 weeks gestation you are 4 weeks pregnant. (ie you are only just skipping your period).
At 4 weeks gestation (6 weeks pregnancy) your period is 2 weeks late.

Incidentally, if someone is trying for a baby and / or tracking their cycle carefully and they test 4 days before their period is due and get a positive (first response) then yes they can find out they are pregnant - just before they are 4 weeks.

I hope this clears any matters up for anyone.