Preheat the oven to 425 degrees and position the rack in the bottom half of the oven. Once the oven reaches temperature, place the popover pan in the oven. In a large bowl, whisk together the eggs, milk, salt, flour and 3 Tbsp. of the melted butter. Mix in the cheese and chives.
Remove the pan from the oven and quickly divide the remaining 1 Tbsp. of melted butter among the pans. Fill the pans with batter and return to the oven. Lower the temperature to 375 degrees and bake, undisturbed, for 45 minutes. Serve warm.