If you series connect them, the total would be 20 ohms. Chances are you ain't gonna do that.
In parallel, the total would be 3.2 ohms. The calc for the total is easier to do if the two cabs are the same value - just divide one by two. But when 2 cabs are different values, you have to take the product of the two and divide that by the sum of the two.
(R1 * R2) / (R1 + R2)
(16 * 4) / (16 + 4)
(64) / (20)
3.2