Whisk the brown sugar, ketchup, barbecue sauce and mustard together in a bowl.

Place the cocktail sausages in a 4-quart crockpot and pour the sauce over them.

Stir gently to coat every sausage.

Cover and cook on low for 3 to 4 hours or high for 2 hours, stirring once halfway through, until the sauce turns glossy and clings to the sausages instead of running thin.

Keep on warm and serve straight from the crockpot with toothpicks.