Have you set up an apt with your dr yet? I would suggest at least calling and letting them know what's going on, they may want to take blood to make sure your numbers are increasing. Since your only 4 weeks 5 days they may not do an ultrasound since they won't see much at all till about 6 weeks.
Also, since your in the 4 weeks, many women will spot or bleed when they expect there period, it may be nothing, however you should always let your dr know of any spotting so they can decide if you need to take it easy or not. A co-worker of mine had the same thing happen when she was about 6 weeks and everything was fine. I had something similar and miscarried.
Try not to stress about it, since it's just a little spotting you should be ok, if it gets worse call the dr right away.
I am now 4 months pregnant with my 4th child and with the last 2 pregnancies I did spot it is fairly common. with this pregnancy i actually bled a 4 weeks (which i thought was my period) and then again at 10 weeks, but these things do happen. all i know is that the only thing that ever puts my mind at ease is seeing the doctor and having those words come from their lips. "everything is alright." I would call the doctor and the main thing that they ask and worry about is if you are cramping which is not a good sign