What are the chances of miscarriage?

I just found out I'm about 4 weeks pregnant and I feel as if I don't want to get too excited about it incase something goes wrong. It seems as if so many people on here have experienced miscarriages. Looking online there seems to be different advice, some sites say it is very common, others say the risk is only less than 15% for under 35s... Also some say you will know if you have miscarried from the bleeding and others only got picked up from no heartbeat on the scan.