I had a bit of spotting around the time my periods would have been for the first 3 months, im at 24 weeks and 3 days today (6 months) and haven't had anymore since. Since you're at 4 weeks it's likely just "breakthrough bleeding" which is what I think I had.

Here's a little blurb about it that I just copied and pasted from a site:

#2: Breakthrough Bleeding

Some women experience what is known as ‘breakthrough bleeding’, during the times when a period would have normally been due. Therefore bleeding would appear at around 4, 8 and 12 weeks of pregnancy. It’s often accompanied by the feeling you would normally associate with your period being imminent, i.e. backache, cramps, a heavy sensation in the pelvis, feeling bloated and ‘off’. Of course, because you are actually pregnant, your period doesn’t arrive (even though you feel like it will). During pregnancy, hormones prevent your period from occurring. Sometimes, if the hormone levels are not yet high enough to stop your period, the result is breakthrough bleeding. It can last for about three months; after this time the placenta begins to take over hormone production from the ovaries. Some women experience breakthrough bleeding throughout the pregnancy, and under the close watch of their carer, have healthy babies. - See more at: http://www.bellybelly.com.au/pregnancy/bleeding-during-pregnancy/

Spotting