I don't know if dietary fiber and soluable fiber is the same thing, but I would assume that it is since the fiber count can't be higher than the total carb count. If that's the case, then the net carb would be 1g (4g carb - 3g fiber = 1g net carb).

I take Nature's Sunshine psyllium, and it has 8g of carbs and 8g of fiber = 0g net carb per one teaspoon. Maybe the 1g of net carb in your supplement comes from some form of sugar for better palatability? Mine tastes really yucky and I have to chug it down with a ton of water or I'd throw up!