For a model with a fourth family of quarks, new sources of flavor changing neutral currents are identified by confronting the unitary $4\ifmmode\times\else\texttimes\fi{}4$ quark mixing matrix with the experimental measured values of the familiar $3\ifmmode\times\else\texttimes\fi{}3$ quark mixing matrix. By imposing as experimental constraints the known bounds for the flavor changing neutral currents, the largest mixing of the known quarks with the fourth family ones is established. The predictions are: a value for $|{V}_{tb}|$ significantly different from unity, large rates for rare top decays as $t\ensuremath{\rightarrow}c\ensuremath{\gamma}$ and $t\ensuremath{\rightarrow}cZ$, the last one reachable at the Large Hadron Collider, and large rates for rare strange decays $s\ensuremath{\rightarrow}d\ensuremath{\gamma}$ and $s\ensuremath{\rightarrow}dg$, where $g$ stands for the gluon field, both processes reachable at the existing B factories.