The effect of the low-symmetry crystal field S4 on the spectral shape of Faraday rotation in ferrites containing tetrahedral Co2+ ions was investigated. The calculated shape of Faraday rotation in 4T1 (4F) becomes broad with increasing a', a parameter proportional to the magnitude of the S4 low-symmetry field, above about a' = 1000cm-1 and has a shoulder or other structure when a' ~ 2000 cm-1. Using the value a' = 2000 cm-1, it is possible to explain satisfactorily the broad shape of the experimentally observed Faraday rotation at about 1.5 μm in a (Y2.8 Gd0.2)(Fe4.6Co0.2Ge0.2)O12 thin film grown by the conventional liquid phase epitaxy (LPE) method.