The longest diagonal is about 8.9 inches. The longest length is going to be between the two longest lengths, the width and the height. The Pythagorean theorem is a^2 + b^2 = c^2. Plug the width and height into a and b of the Pythagorean equation. (4)^2 + (8)^2 = c^2. Now simplify the equation. 16 + 64 = c^2. 80 = c^2. Now find the square root of each side. (sqrt)80 = (sqrt)c^2. If you simplify this, you will find that c is equal to about 8.9 inches.
