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A simple Pati-Salam SU(4) model with a low symmetry breaking scale of about1000 TeV is presented. The analysis concentrates on calculating radiativecorrections to tree-level mass relations for third generation fermions. Thetree-level relation $m_b/m_{\tau}=1$ predicted by such models can receive largeradiative corrections up to about $50\%$ due to threshold effects at the massunification scale. These corrections are thus of about the same importance asthose that give rise to renormalisation group running. The high figure of$50\%$ can be achieved because 1-loop graphs involving the physical chargedHiggs boson give corrections to $m_{\tau}-m_b$ that are proportional to thelarge top quark mass. These corrections can either increase or decrease$m_b/m_{\tau}$ depending on the value of an unknown parameter. They can also bemade to vanish through a fine-tuning. A related model of tree-level$t$-$b$-$\tau$ unification which uses the identification of SU(2)$_R$ withcustodial SU(2) is then discussed. A curious relation $m_b \simeq \sqrt{2}m_{\tau}$ is found to be satisfied at tree-level in this model. The overallconclusion of this work is that the tree-level relation $m_b = m_{\tau}$ at lowscales such as 1000 TeV or somewhat higher can produce a successful value for$m_b/m_{\tau}$ after corrections, but one must be mindful that radiativecorrections beyond those incorporated through the renormalisation group can bevery important. This motivates that an on-going search for the rare decays$K^0_L \to \mu^{\pm} e^{\mp}$ be maintained.Comment: 33 pages text plus 7 figures, LaTeX, uses epsf.te

Prospects for mass unification at low energy scales