In a skillet bring 3 cups of the broth to a boil, add the chicken, reducing the heat to keep the broth at a bare simmer, ad simmer it, turning it once, for 8 minutes, or until it is springy. Transfer the chicken to a cutting board, reserving the broth, and chop it.
In a large saucepan cook the onions, the celery, and the apple in the oil, covered, over moderately low heat, stirring occasionally, for 10 minutes, or until the vegetables are softened, stir in the curry powder and cook the mixture, stirring, for 3 minutes, and stir in the remaining 1 cup broth. Bring the liquid to a boil and simmer the mixture, stirring occasionally, for 10 minutes.
In a food processor or blender puree the mixture, transfer the puree to a bowl, and stir in the reserved broth and the chicken. Let the mixture cool, stir in half-and-half, and chill the soup, covered, for 3 hours, or until it is cold. Garnish the soup with the raisins.