Could there be a Fourth Generation of Quarks Without More Leptons?
  We investigate the possibility of adding a fourth generation of quarks. We also extend the Standard Model gauge group by adding another SU(N) component. In order to cancel the contributions of the fourth generation of quarks to the gauge anomalies we must add a generation of fermions coupling to the SU(N) group. This model has many features similar to the Standard Model and, for example, includes a natural generalisation of the Standard Model charge quantisation rule. We discuss the phenomenology of this model and, in particular, show that the infrared quasi-fixed point values of the Yukawa coupling constants put upper limits on the new quark masses close to the present experimental lower bounds. 
C. D. Froggatt, H. B. Nielsen and D. J. Smith