  In the scenario with four quark generations, we perform a fit using
flavor-physics data and determine the allowed values -- preferred central
values and errors -- of all of the elements of the 4X4 quark mixing matrix. In
addition to the direct measurements of some of the elements, we include in the
fit the present measurements of several flavor-changing observables in the K
and B systems that have small hadronic uncertainties, and also consider the
constraints from the vertex corrections to Z -> b bbar. The values taken for
the masses of the fourth-generation quarks are consistent with the measurements
of the oblique parameters and perturbativity of the Yukawa couplings. We find
that |{\tilde V}_{tb}| >= 0.98 at 3sigma, so that a fourth generation cannot
account for any large deviation of |{\tilde V}_{tb}| from unity. The fit also
indicates that all the new-physics parameters are consistent with zero, and the
mixing of the fourth generation with the other three is constrained to be very
small: we obtain |{\tilde V}_{ub'}| < 0.06, |{\tilde V}_{cb'}| < 0.027, and
|{\tilde V}_{tb'}| < 0.31 at 3sigma. Still, this does allow for the possibility
of new-physics signals in Bd, Bs and rare K decays.
