Spoon yogurt onto several layers of heavy-duty paper towels, and spread to 1/2-inch thickness. Cover with additional paper towels; let stand 5 minutes. Scrape yogurt into a medium bowl, using a rubber spatula.
Add chicken and next 5 ingredients to yogurt; stir well. Place lettuce leaves over 4 bread slices; top with chicken mixture, tomato slices, and remaining bread slices.