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°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 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.
The brittle can be stored in an airtight container at room temperature for up to 1 month.