In a 2-quart saucepan, combine the sugar, water, butter and corn syrup and bring to a boil to create caramel. Cook over moderately high heat, stirring occasionally, until the caramel is light brown and registers 300° F. on a candy thermometer, about 10 minutes. Remove from the heat and carefully stir in the baking soda. The mixture will bubble. Stir in the nuts, then immediately scrape the brittle onto the prepared baking sheet. Using the back of a large spoon (oil it lightly if it sticks), spread the brittle into a thin, even layer (see *Tip). Let cool completely, about 30 minutes.