fcc packing has an abcabc arrangement of close packed planes

The structure contains octahedral and tetrahedral holes. For n spheres there are n octahedral holes and 2n tetrahedral holes.

Each sphere on a vertex of the unit cell contributes 1/8 (it is shared between 8 unit cells), and each sphere in the centre of a face contributes 1/2 (it is shared between 2 unit cells)

Therefore the total number of spheres in the unit cell is:

8x(1/8) + 6x(1/2) = 4