brown is always old blood so i would consider it just left over period blood.. although it is possible to get preg on that day it is very unlikely... i consider the day after my period as a sfe day for me... hasnt failed me yet... inmplantarion would not happen at four days... but if you are still worried you can get an accurate test result 14 days after your unprotected sex... i personally would not worry about it
Implantation typically occurs 6-10 days after unprotected sex
You had your period 4 days ago and you had sex during your period? Its always a possibility to be pregnant when you have unprotected sex; however you are probably still experiencing your period; just the last of it.
More then likely all it is is old blood.. Nothing to be concerned about.