At 4 weeks it's possible it's breakthrough bleeding from your period. I bled for a few days at exactly 4 weeks with both of my pregnancies because of breakthrough bleeding, but it was fine. The only time spotting turned out to mean I had a problem was when I spotted at 11 weeks.
Spotting that is brown or light pink is normal (: it's implantation bleeding. Expect to see it in the early stages of pregnacy. If it's super red, almost period like contact your ob or go to the er cause that could be something else.