In a heavy saucepan heat sugar, syrup, water and salt to a rolling boil. Add peanuts. Reduce heat to medium and stir constantly. **Cook to hard crack stage (293° to 300° F.) Add butter, then baking soda. Beat rapidly and pour on a buttered surface spreading to 1/4-inch thickness. When cool break into pieces. Store in an airtight container. Makes about 2 pounds. **Note: In the absence of a candy thermometer, test for hard crack stage by dropping a tiny bit of syrup into ice water (let it thread from a spoon into the water). When the threads are brittle (not pliable) it has reached the hard crack stage.