I'm not sure how accurate this is but this is how I always determined when my fertile days were. Someone once told me that you find the first day you started your period and then count from there and the 13th, 14th and 15th days are your most fertile. So if you started your period on 4/1 then 4/13, 4/14 and 4/15 are your fertile days. Hope this helps.
Yeah, 4 weeks in gestational age is about 2 weeks from the time of conception. So you probably got pregnant a couple of weeks ago.