In a heavy 5- to 6-quart pan over medium-high heat, stir 4 cups whole milk and 1 1/4 cups sugar until sugar is dissolved and mixture is boiling. Stir in 1/2 teaspoon baking soda. Reduce heat to low and simmer, stirring occasionally with a flexible spatula, until mixture is golden brown and reduced to about 2 cups, about 1 1/2 hours (mine took about 2 1/2 hours on very low heat).
Pour through a fine strainer into a bowl; discard residue. (*I did not have to do this step. Mine was smooth and rich and had no residue.)