Preheat oven to 350°F (175°C). Grease a 9x13-inch baking pan.
In a large bowl, combine the cake mix, 3 eggs, melted butter, and 1/4 teaspoon salt. Mix until smooth. Pour this batter into the prepared pan and spread evenly.
In another bowl, beat the cream cheese, sugar, remaining 1 egg, and vanilla extract until smooth and creamy.
Pour the cream cheese mixture over the cake batter in the pan, spreading it evenly without mixing through.
Bake for 35–40 minutes, or until the top is lightly golden and a toothpick inserted near the center comes out clean.
Cool completely before slicing. Serve at room temperature.
Notes
Best served fresh and at room temperature. Store leftovers covered in the refrigerator for up to 3 days.