Answer:
108 cubes
Explanation:
The volume of a cube is given by the formula:

Where
V is the volume, and
s is the side length
Given side length of the cube as 1/3, we can find it's volume:

So, each cube has volume of 1/27
The rectangular prism's volume is 4. So the number of cubes that will fit the rectangular prism is:
Number of Cubes = 4/(1/27) = 4 * 27 = 108 cubes