In a large saucepan, combine the sugar, water, butter and corn syrup and bring to a boil. Cook over moderately high heat, stirring occasionally, until the caramel is light brown and registers 300° on a candy thermometer, 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 a large rimmed, nonstick baking sheet. Using the back of a large spoon (oil it lightly if it sticks), spread the brittle into a thin, even layer. Sprinkle with salt. Let cool completely, about 30 minutes. Break the brittle into large shards.