4 weeks pregnant and really nervous!?

So i just found out im 4 weeks pregnant, this is my second baby and so many people around me have had miscarriages with their second children that I know. I'm terrified thats going to be me. I'm trying so hard to pray and be calm about it but my sister in law is pregnant with her first and due 3 days after me and I'm so terrified i'm gonna lose mine and have to watch her be pregnant and carry to full term. Are most miscarriages spontaneous and because the chromosomes don't fuse together right or what? I'm just so nervous because i'm only feeling a tad crampy and only some days..any kind words of reassurance would be greatly appreciated!