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We estimate the CP asymmetry A_{CP}(q^2) in the decays B --> X_s mu+ mu- andB --> X_d mu+ mu- in the standard model (SM) with an additional fourthgeneration. We use a parametrization that allows us to explore the completeparameter space of the 4X4 quark mixing matrix, and constrain these parametersfrom the current data on B decays. We find that the enhancement in A_{CP}(q^2)depends strongly on the mass of the t', the up-type quark in the fourthgeneration. For m_t' around 400 GeV, the CP asymmetry in the high-q^2 region(q^2 > 14.4 GeV^2) can be enhanced by more than an order of magnitude for B -->X_s mu+ mu- and up to a factor of 6 for B --> X_d mu+ mu-. There is noenhancement in the low-q^2 region (1< q^2 <6 GeV^2). With increasing m_t', theA_{CP}(q^2) in the high-q^2 (low-q^2) region first decreases (increases) andthen saturates at a value a few times the SM prediction. In the high-q^2 regionof B --> X_s mu+ mu-, this saturation value may be up to 25 times the SMexpectation.

CP asymmetry in the decays B --> (X_s, X_d) mu+ mu- with four generations