Mix salsa and cumin. Set aside. Butter 1 side of each of the bread slices. Layer 2 slices of cheese on each of 4 bread slices with buttered side down. Spread with salsa mixture. Top with remaining bread slices with buttered side up.
Place sandwiches in large skillet or griddle on medium-low heat. Cook 3 to 4 minutes per side or until bread is browned and cheese is melted. Cook in batches if necessary.