Combine water, 3 cups broccoli, celery, onion, carrot, garlic, rosemary, and thyme in a large pot over medium heat. Cook until vegetables are tender, 20 to 30 minutes. Add remaining 1 cup broccoli and continue cooking until just tender, about 5 minutes more. Remove soup from heat.
Puree soup using an immersion blender. Return pot to medium heat. Add barley and coconut milk. Cook until barley is tender but still firm to the bite, about 10 minutes more. Season with salt and pepper.