Fe³⁺ has 3d⁵ outer electronic configuration.
CN⁻ ion is a strong field ligand. It produces a low spin complex with 1 unpaired electron having a magnetic moment.
μeff = √n(n+2) = √1(1+2) = √3 = 1.73 B.M.
SCN⁻ ion is a weak field ligand. It produces a high spin complex with 5 unpaired electrons having a magnetic moment.
μeff = √n(n+2) = √5(5+2) = √35 = 5.92 B.M.
Hence, the difference in the magnetic moment is 5.92 B.M. - 1.73 B.M. = 4.19 B.M. ≈ 4 B.M.