In a mixing bowl, combine ricotta, mozzarella, sun-dried tomatoes, and spinach. Season with salt and pepper.
Spread the cheese mixture over 4 slices of bread and top with the remaining bread slices.
Butter the outside of each sandwich on both sides.
Heat a large non-stick skillet over medium heat. Grill sandwiches until golden brown and cheese is melted, about 3–4 minutes per side.
Serve warm and enjoy!