Gauge-coupling unification and the minimal SUSY model: a fourth generation below the top?

\centerline{\bf Abstract} We explore the possibility of a fourth generation in the gauge-coupling-unified, minimal supersymmetric (MSSM) framework. We find that a sequential fourth generation (with a heavy neutrino \nu') can still fit, surviving all present experimental constraints, provided \lambda_b(M_U)=\lambda_\tau(M_U) Yukawa unification is relaxed. For the theory to remain perturbative up to M_U, the new leptonic generation must lie within reach of LEP-II and the new b',t' must have masses within the reach of the Tevatron. For example, for \mt>150\gev we find m_{\nu'}, m_{\tau'}< 86\gev, m_{t'}<178, and m_{b'}<156\gev. Experiments at Fermilab are already sensitive to the latter mass regions; we comment on direct b' searches and on the m_{t'}\simeq \mt case in light of new CDF data. Discovery may involve novel decay signatures; however, CDF and LEP-II will confirm or exclude an MSSM fourth generation in the near future.