Melt 2 tablespoons of the butter in the saucepan. Add the chopped garlic and cook over moderate heat until softened. Add the baby peas and toss to coat. Add the couscous, the remaining 2 tablespoons of butter and 1 cup of the couscous cooking liquid and simmer, stirring occasionally, until the couscous and peas are tender and coated in a creamy sauce, 3 to 4 minutes. Season the couscous and peas with salt and pepper and stir in the mint. Stir in some of the remaining cooking liquid if the couscous seems dry. Transfer the couscous to a bowl and serve.