Each cube is six such squares with a surface area of 6X 400 = 2400 square centimeters.

The sum of the surface areas of four identical cubes is 4X 2400 = 9600 square centimeters.

When four identical cubes are put together, they are either a strip or a "field".

The largest surface area means the least overlapping squares, which are a long strip with three overlapping places in the middle, reducing six squares.

9600-6X 400 = 9600-2400 = 7200 square centimeters

The smallest surface area means the largest number of overlapping squares, just like a "field", where there are four overlapping places in the middle, reducing 8 squares.

9600-8X 400 = 9600-3200 = 6400 square centimeters