In a medium saucepan, bring 4 1/4 cups water and the salt to a boil. Whisking constantly, slowly add the grits in a steady stream. Reduce heat to low. Cook, stirring frequently, until the grits are thick, creamy, and pulling away from the side of the pan, 15 to 20 minutes. Add the Cheddar and stir until it melts. Add the scallions, pepper, and butter and stir.