In a small bowl, stir together 1/4 cup Greek yogurt (or sour cream), 1/4 cup light ranch dressing, 1 tablespoon minced cilantro or parsley, and salt and pepper to taste. Taste and adjust seasoning if needed; set the sauce aside.
If your cooked chicken is not already chopped or shredded, chop or shred it so it’s ready to use.
Slice the avocado (peeled and pitted) and the tomato. If using cheese, have the 1/4 cup shredded cheddar or mozzarella ready.
Lay the four medium flour tortillas flat on a clean work surface.
Divide the 2 cups cooked chicken into four equal portions (about 1/2 cup per tortilla). Place one portion down the center of each tortilla.
Divide the avocado slices and tomato slices evenly among the tortillas and arrange them on top of the chicken.
If using cheese, sprinkle the 1/4 cup shredded cheese evenly over the fillings (about 1 tablespoon per tortilla).
Divide the yogurt–ranch sauce evenly among the four tortillas (about 2 tablespoons per tortilla). Drizzle or spoon the sauce over the fillings.
Fold the sides of each tortilla in toward the center, then roll tightly from one end to the other to form burrito-style wraps. Secure with a toothpick if needed and serve immediately.