Preheat oven to 325 degrees. Separate eggs and reserve whites. Beat 4 yolks, add sweetened condensed milk and lime juice, beat until thick, and pour into pre-baked pie shell. Blend 4 egg whites with sugar and cream of tartar; beat until stiff peaks are formed and spread over surface of pie. Bake at 325 degrees until topping is golden brown. Cool completely before cutting. Serve garnished with thin slices of lime.