Cook the pasta according to the package directions; drain.
Meanwhile, heat 2 tablespoons of the butter in a large skillet over medium heat. Add the carrots, rosemary, and ¾ teaspoon each salt and pepper. Cook, stirring often, until tender, 12 to 15 minutes.
Add the pasta, lemon juice, and the remaining 2 tablespoons of butter to the carrots and toss to combine.
Serve the pasta topped with grated Parmesan.