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 $4\times 4$ 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 $Z\to b\overline{b}$. 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 $|{\tilde{V}}_{tb}|\ge 0.98$ at $3\sigma$, so that a fourth generation cannot account for any large deviation of $|V_{tb}|$ 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 small: we obtain $|{\tilde{V}}_{u{b}^{\prime }}|<0.06$, $|{\tilde{V}}_{c{b}^{\prime }}|<0.027$, and $|{\tilde{V}}_{t{b}^{\prime }}|<0.31$ at $3\sigma$. Still, this does allow for the possibility of new-physics signals in $B_d$, $B_s$ and rare $K$ decays.

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