In a large skillet, melt 2 tablespoons of the butter over moderate heat. Add the onion and garlic and cook, stirring occasionally, until lightly browned, about 10 minutes. Transfer the onion and garlic to a food processor. Melt the remaining 2 tablespoons of butter in the same skillet. Add the chicken livers, season with salt and 1/2 teaspoon of quatre-épices and cook over high heat until nicely browned on the outside and still pink inside, about 5 minutes. Transfer the livers to the food processor.