There are 8 corners of the unit cell and each corner has one atom. each atom is shared by 8 units cells.
total no. of atom at corner = 8*1/8 = 1 atom
there are 6 faces which have one atom in the centre of it.
each such atom is electron is shared between 2 unit cell
total no. of atom at centre of faces = 6*1/2 = 3 atom
so total atom = 1+3 = 4 atoms