Preheat the grill to medium-high heat. Form the ground beef into 4 equal-sized patties and sprinkle with the salt and pepper. Cook the patties on the grill for 10 to 12 minutes, or until no pink remains in the center, turning once halfway through the grilling. Meanwhile, in a small bowl, combine the mustard, chives, and garlic; mix well. Spread the mustard mixture evenly over the crusty rolls. Just before the burgers are finished cooking, place a wedge of Brie over the top of each burger, allowing it to melt slightly. Please a burger on each roll and serve immediately.