I had some very intense back pain when I was about 4 weeks along as well, and it terrified me. I thought if I was hurting so bad this early into the pregnancy, there's no way I'd last through to 9 months. Thankfully, this pain only lasted a couple of days for me, and my back feels pretty good now. Maybe it was my body's reaction to the initial release of hormones, I'm not sure. But I hope you have the same experience I had and it just decides to go away as quickly as it came. Good luck!