Got period but light, got it for 4 days can i be pregnant ? when can i take preganancy test ? how accurate it will be?

Tests are accurate: It is hard to guess whether or not a woman is newly pregnant. A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), 1 week after missing a period, or 1 week after starting a "weird" period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon.