Z4symmetry and the fourth generation

The $Z_4$ horizontal symmetry of weak interactions is used to study the masses and mixings of a possible fourth generation of quarks (t’,b’). We obtain the relation ($m_s$/$m_c$)=($m_b$+$m_{b\mathcal{’}}$)/($m_t$+m ${}_{t\mathcal{’}}{}^{}{}_{}{}^{}$) which implies $m_{b\mathcal{’}}$/$m_{t\mathcal{’}}$≪1. Coupled with the known experimental constraints on the ρ parameter, this leads to $m_t^{\mathcal{’}}$≤300 GeV and $m_{b\mathcal{’}}$<58 GeV. We also predict the mixing of the fourth-generation quarks to those of the known quarks.