check the picture below.
so, notice there, the rhombus is really just two triangles, each triangle has a base of 4 units and a height of also 4 units, so simply let's get the area of a triangle and multiply that times 2.
[tex]\bf \stackrel{\textit{area of 2 triangles}}{2\left[\cfrac{1}{2}(4)(4) \right]}\implies 16[/tex]
