600 calories is barely above starvation, there is no way you can eat that little for a period of time and not lose weight, I dont care what your genetics are its just basic pyhsics you would be putting in less energy than you are putting out just in heat and basic movement etc.
You've made a mistake there.
Anyway, getting away from that, congrats on the 4lbs, I hope you can keep it up.
More exercise is a far healthier way to lose weight than less food.