Place the polenta and 4 cups of water in a heavy-bottomed 2½-quart saucepan (preferably one with flared sides) and stir to combine. Set the pan over medium-high heat and bring to a simmer, stirring constantly with a wooden spoon, until the first starch takes hold, 5 to 8 minutes. Reduce the heat to low and cook, stirring frequently, until the grains are soft and hold their shape on a spoon, about 1 hour. Whisk in the salt, pepper, butter, and Parmesan. Serve hot. (To keep the polenta hot for up to 30 minutes before serving, transfer it to a bowl, cover, and set the bowl over a saucepan of barely simmering water. If necessary, thin the polenta with hot water before serving.)