Split each sweet potato down the center lengthwise, but do not cut all the way through. Place each sweet potato on a double layer of heavy-duty aluminum foil and fill with 2 tablespoons butter per potato. Sprinkle 1/4 of the brown sugar on top of each potato. Fold the foil securely around each potato and twist the ends to prevent the butter from leaking out. Place the potatoes in hot coals for approximately 1 hour, or until soft. Unwrap the foil, place a potato on a plate, and pour the melted butter and brown sugar back onto the potato.