The simple cubic unit cell is delineated by eight atoms, which mark the actual cube. These are corner atoms, so each one only contributes one eighth of an atom to the unit cell, thus giving us only ...

occupying the octahedral holes. Notice that the octahedral holes are along the edges (for a net of three cations) and in the center of the cube (one more cation). Thus we have four anions (FCC) and ...