Using a pastry brush, butter each slice of bread on one side, and place 4 slices, buttered side down, on a work surface. Place on each of these slices of Cheddar or Monterey Jack cheese. Top with the remaining 4 bread slices, buttered side up.
Lightly butter and heat a griddle, heavy skillet or Panini press until hot. Place 2 sandwiches in the skillet and weight with another heavy pan or skillet. Brown slowly on one side for 2-3 minutes before turning, with a spatula, and grilling the other side until brown and cheese melts. It may be necessary to add a bit of butter to the skillet.
Repeat with remaining two sandwiches. Serve immediately.