Heat the oven to 350°F/180°C.
Melt 2 tablespoons of the butter in sauté pan and gently sauté the onion until soft. Add the garlic one minute, then pour in the Cognac, remove from the heat, and cool slightly.
Combine the pork, veal, and liver in a large bowl. Add the breadcrumbs, herbs, quatre-épices and eggs. Mix well. Season well with salt and pepper. Fry a patty of it to check for seasonings and adjust if necessary.
Thinly slice the pork fat and line a terrine mold with the slices, slightly overlapping them and allowing it to hang over the edges. Pack the meat mixture into the terrine and press down evenly. Fold the overhanging strips of bacon over the top and add a few more slices if necessary to cover the top. Cover the top with a piece of parchment paper and then cover the terrine with foil.
Put the terrine in a roasting pan and pour around enough boiling water to come halfway up the sides of the terrine. Place in oven and cook for 1 to 1-1/2 hours. Remove from the water bath and cool completely. Place a brick on it refrigerate overnight.