Use 1 tablespoon of the butter to generously grease a shallow 3-quart casserole, Dutch oven or baking dish. Melt the remaining 3 tablespoons butter in a large pot over medium heat. Add the onion, celery, and 1-1/2 teaspoons salt. Cook, stirring occasionally, until translucent and just tender, about 4 minutes. Add the poultry seasoning and cook, stirring for 1 minute.
Add the milk, sugar and 1 cup water, and bring to a boil. Continuously whisk the mixture while you pour in the cornmeal in a slow, steady stream. Keep whisking while the mixture bubbles rapidly until the cornmeal has absorbed all the liquid and is thick and smooth, about 5 minutes. Remove from the heat and stir in the corn and baking powder. Cool, stirring often, until lukewarm. Don’t let the mixture clump.
Whisk the eggs in a large bowl until pale yellow and very foamy with no liquid remaining. Add one third of the beaten eggs to the cornmeal mixture and stir to loosen the cornmeal mixture. Add half of the remaining eggs and fold gently until incorporated, then repeat with the remaining eggs. Spread evenly in the prepared dish.
Bake until golden brown and set without any jiggling, about 25 minutes. When you press the top, it should spring back. Serve hot or warm.