  We study the effects of sequential fourth quark generation in rare "B ->
K(K*) l^+ l^-" decays induced by "b -> s" transition and in "B_s^0 -
bar{B}_s^0" mixing. Using the experimental values on branching ratios of the "B
-> X_s gamma" and "B -> K(K*) l^+ l^-" decays, the allowed regions for $\vel
V_{tb} V_{ts}^\ast\ver$ and "|V_{t'b} V_{t's}*| are determined as a function of
the (t') quark mass.
