Brown means old so brown blood isn't necessarily the start of a miscarriage. It could be old blood from implantation just working it's way out. Did they tell you what your hCG numbers were? You are still awfully early (4 weeks 2 days, if I read right?) and depending on a whole host of things (ovulation, implantation, etc.) your numbers probably won't be all that high right now. The 'normal' range for hCG at 4 weeks is 5-426 mIU according to the American Pregnancy Association. Here is a full list of the 'normal' range for a pregnancy from their website.
hCG levels in weeks from LMP (gestational age)* :
3 weeks LMP: 5 – 50 mIU/ml
4 weeks LMP: 5 – 426 mIU/ml
5 weeks LMP: 18 – 7,340 mIU/ml
6 weeks LMP: 1,080 – 56,500 mIU/ml
7 – 8 weeks LMP: 7, 650 – 229,000 mIU/ml
9 – 12 weeks LMP: 25,700 – 288,000 mIU/ml
13 – 16 weeks LMP: 13,300 – 254,000 mIU/ml
17 – 24 weeks LMP: 4,060 – 165,400 mIU/ml
25 – 40 weeks LMP: 3,640 – 117,000 mIU/ml
Non-pregnant females: <5.0 mIU/ml
Postmenopausal females: <9.5 mIU/ml
* These numbers are just a GUIDELINE– every woman’s level of hCG can rise differently. It is not necessarily the level that matters but rather the change in the level.
Notice it says it's not the level but rather how it changes. I hope this helps! And as long as you aren't cramping and the discharge stays brown, try not to worry. I know it's hard not to worry when you see stuff like that but try to relax as best you can, push fluids, and keep yourself occupied. Staying busy has helped me thru several miscarriages and two successful pregnancies. It's also keeping me sane now since I found out about my most recent pg on Sunday (just 5 weeks after my last miscarriage). Good luck and I hope your repeat labs show a nice rise in hCG!