In a large saucepan, heat 1 tablespoon of the olive oil. Add the leeks, garlic, potato and fennel. Cover and cook over low heat until tender, about 10 minutes. Add the romaine and Bibb lettuces and toss just until wilted, about 30 seconds. Transfer the contents of the saucepan to a blender. Add the vegetable stock, parsley and the remaining 3 tablespoons of olive oil and puree. Strain the soup through a fine sieve into a medium bowl. Stir in the lemon juice and season with salt and pepper. Refrigerate the lettuce soup until chilled.