We examine conditions for models with fourquark interaction and a finite cutoff to be a reasonable approximation to the QCD at low energies. The flavourdependent part of QCD vacuum energy is identified with an effective potential for quark model. The independence on the cutoff and the scale anomaly of QCD is exploited in the quark sector to establish the scaling law for QCD-motivated effective coupling constants. On the contrary the insensitivity of effective potential in respect to "intrinsic" field dilatations leads to a selection rule for parameters of quark models to make a truncation of QCD.