Spreading a little mayonnaise over the outside of the bread before grilling adds golden color and delicious crunch.
Cook bacon in heavy large skillet over medium heat until brown and crisp, turning occasionally, about 6 minutes. Transfer bacon to paper towels and drain. Wash and dry skillet.
Place 4 bread slices on work surface. Press 1/4 cup grated cheese onto each slice. Top each with 2 tomato slices. Sprinkle with pepper. Place 2 bacon slices atop each, breaking into pieces if necessary to fit. Press 1/4 cup grated cheese over bacon on each. Top sandwiches with remaining bread slices, then spread 1/2 tablespoon mayonnaise over top of each sandwich. Heat 2 heavy large skillets over medium heat. Add 2 sandwiches, mayonnaise side down, to each skillet. Place plate atop both sandwiches to weigh down. Cook sandwiches until bottom is golden brown, about 2 minutes. Spread top of each sandwich with 1/2 tablespoon mayonnaise. Turn sandwiches over, mayonnaise side down. Top with plates and cook until golden brown on bottom, about 2 minutes. Transfer sandwiches to work surface. Cut sandwiches crosswise in half and serve.