Brown spotting on the day of your period during week 4 can be a common occurrence and does not necessarily indicate that you have not conceived. It is important to understand that every woman's menstrual cycle can vary, and experiencing brown spotting instead of a regular flow is not uncommon. This could be due to various factors such as hormonal changes, implantation bleeding, or even just a lighter period. If you are actively trying to conceive and are concerned about your fertility, it is recommended to consult with a healthcare professional who can provide personalized advice based on your specific situation. They may suggest further tests or evaluations to determine any underlying causes or address any concerns you may have.