Cook yams in boiling water for 25-35 minutes or until very soft and tender. Drain and mash thoroughly until consistency is smooth.
Preheat oven to 400 degrees. Mix drained pineapple and 3 tablespoons syrup into mashed yams. Spoon into a 9 inch round or 8 inch square non-stick baking pan. Beat egg whites to soft peaks; add remaining 1 tablesppon syrup and beat until stiff; spread with a spatula, using a swirling motion, on top of yam mixture. Bake at 400 degrees for 8 to 10 minutes, or until top is golden.