Fourth generation effects in rare (B_s -> l^+ l^-) and (B_s -> l^+ l^- gamma) decays

The contribution of the fourth generation quarks to the rare (B_s -> l^+ l^-) and (B_s -> l^+ l^- gamma) decays are studied. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix elements |V_{tb} V_{ts}*| and |V_{t'b} V_{t's}*| are obtained from the existing experimental result for the branching ratio of the (B -> X_s gamma) decay. The branching ratios of the above-mentioned decays, as well as the photon spectrum in the (B_s -> l^+ l^- gamma) decay, are studied. It is found that the results are sensitive to the existence of the fourth generation.