Bounds on fourth generation induced Lepton Flavour Violating double   insertions in Supersymmetry
  We derive bounds on leptonic double mass insertions of the type $\delta^{l}_{i4} \delta^{l}_{4j}$ in four generational MSSM, using the present limits on $l_i \to l_j + \gamma$. Two main features distinguish the rates of these processes in MSSM4 from MSSM3 : (a) tan$\beta$ is restricted to be very small $\lesssim 3 $ and (b) the large masses for the fourth generation leptons. In spite of small $\tan\beta$, there is an enhancement in amplitudes with $llrr$($\delta_{i4}^{ll}\delta_{4j}^{rr}$) type insertions which pick up the mass of the fourth generation lepton, $m_{\tau'}$. We find these bounds to be at least two orders of magnitude more stringent than those in MSSM3. 
Sumit K. Garg and Sudhir K. Vempati
hep-ph