1. Preheat oven to 350 degrees F. Rub extra papery skin off of whole heads of garlic. Cut tips off each clove of garlic with serrated knife to exposing a bit of garlic. Lay root side down on a sheet of aluminum foil or in a small baking dish. Drizzle 2 tablespoons oil over the cut end of the garlic. Crimp foil closed or cover the baking dish with foil. Roast until the garlic cloves are soft and fragrant, 45 minutes to 1 hour. Open foil and let sit until cool enough to handle.
2. Squeeze the garlic cloves out of their papery skin. Discard skin and transfer the cloves to a food processor. Add the remaining 2 tablespoons oil, chickpeas, lemon juice, tahini and salt and puree until completely smooth, about 1 minute. Serve sprinkled with sumac, parsley and drizzled with more olive oil if desired.smooth, about 1 minute. Serve sprinkled with sumac, parsley and drizzled with more olive oil if desired.