In a large saucepan, melt 2 tbsp. butter over medium heat. Add the onion and cook, stirring, 1 minute. Stir in the barley and wine and simmer until the liquid is absorbed, about 3 minutes. Add the broth and 2 cups water and bring to a boil. Cover, reduce heat to low and cook until the barley is tender and the liquid is absorbed, about 45 minutes.
Stir the shrimp into the barley. Cover and cook until the shrimp are firm and pink, 4 to 5 minutes. Stir in the remaining 2 tbsp. butter and the crushed red pepper; season with salt and pepper. Sprinkle with the rosemary and lemon zest. Serve with the lemon wedges.