Ok, there was totally not enough information in that question.

However, NO it is not possible to test positive 4 days after sex.

the hormone hCG that makes a pregnancy test show positive results normally takes you being at least 3 weeks pregnant for most at home pregnancy tests.

Actually, the HCG can be detected in a woman's body 10-14 days after conception. 3 weeks would mean your period was a week late by the time you tested. You can test as soon as you miss your period.