To find the volume we use an equation:
volume = length x width x height
Since this shape is a cube, you only need to know one side to find the volume since all of the sides are equal.
So all you need to find the volume is to multiply 4(1/2) which I am going to simplify to 4.5, is plug it into the equation.
volume = 4.5 x 4.5 x 4.5
Which will get you an answer of 91.125 inches cubed
Hope this helps :)