In a face centred cubic cell, the contribution of an atom at a face of the unit cell is:

An FCC unit cell contains four atoms: 

Eight atoms present at each corner of cube, one-eighth of an atom at each of the eight corners

$\frac{1}{8}\times 8=1$

There are six faces and each faces corresponds to an atom, the contribution of each atom at each face is $\frac{1}{2}.$

$6\times \frac{1}{2}=3$

atoms present at each corner of cube, one-eighth of an atom at each of the eight corners

Total atoms in FCC=1+3=4.