Hi! In TCB wedding cake section there is a table (P. 513) that lists frosting quantities. Since you already have filling, but do plan a 4” high cake (the chart assumes 1.75” high cake)—I think you’d probably want about 8 cups…if you plan to do additional decor, go with about 10 cups. It freezes well so extras needn’t be wasted. Do you plan to do the same with the 12x9”? The entry for the chart seems to have an error…as it’s listed at 1.75” high but has filling—anyway, the amount she lists is about 4.5 cups for the 9 x13” filled but only 1.75” high… so if you do about 15/16 cups I think you’d be set for both cakes.